Sonic the Hedgehog CD (also known as Sonic CD) is a Sonic the Hedgehog game released for the Sega CD in 1993 as a direct sequel to the original 1991 game. While it was released after Sonic the Hedgehog 2, the games were developed in tandem, and as a result, have some notable differences between them; the Compilation Rerelease Sonic Origins would later definitively confirm that CD takes place before Sonic 2, helping explain at least a few of the discrepancies. This game introduced two Breakout Characters: Sonic's pink fangirl Amy Rose, and his definitive Evil Knockoff Metal Sonic. The animated opening and ending sequences were done by Studio Junio and Toei Animation.

For one month out of the entire year, Little Planet — a tiny planet with an unusual orbit — appears over Never Lake. Little Planet is the home of seven miraculous Time Stones that have been reported to cause wonders, such as preserving the natural world from the passage of time.

To witness this phenomenon, Sonic journeys to Never Lake, but is shocked to find Little Planet encased in a metallic shell and chained to a mountain — one carved with the distinct face of Dr. Eggman (also known as Dr. Ivo Robotnik). The scientist seeks to halt Little Planet's orbit and use the Time Stones to rule over time itself.

Meanwhile, Amy Rose — who predicted that Sonic would be arriving at Never Lake by reading Tarot cards — tags along and gets kidnapped by Sonic's robotic doppelgänger, Metal Sonic. It's up to Sonic to save Amy, the future of Little Planet, and the entire timeline.

The gimmick of the game is time travel, by which you can save Little Planet's future: each Round on Little Planet has three Zones, the first two of which have strategically-placed "Past" and "Future" signs. If you hit one of those signs, you'll have a brief window of time to gain enough speed to hurtle Sonic into either the past or the future. By default, the future is always bad: a neglected, decaying ruin left behind after Eggman has conquered and exploited the Zone. You'll want to avoid those if possible — it's possible to hit a Future sign and get sent there by accident!

However, by destroying Eggman's robot teleporters in the past, the future automatically becomes good: all advances in technology will be working to the environment's benefit, and the robots in all three time periods are destroyed (mostly; some will remain in the present). You can also optionally destroy Metal Sonic holograms to bring back the local fauna in each Zone.

Alternatively, you can go to the Special Stages to collect Time Stones; grabbing them all automatically gives every Zone a good future. There is no further need to travel in time, as this will also destroy all of the transporters in the game. The third Zone of each Round is a boss stage, and takes place in the future you created. The bosses are quite different from those in the first two games: the difficulty comes not from fighting the boss, but figuring out how to harm it.

Sonic has two new moves: the Spin Dash — a move carried over from Sonic 2 that lets Sonic use his trademark Rolling Attack without running first — and the "Super Peel Out", a technique which sends him blasting off at even higher speeds, but leaves him vulnerable while revving up. The Spin Dash operates very differently than in subsequent games, again because it was developed in parallel with Sonic 2; it has to be "revved up" for a second before you can go.

A notable aspect of Sonic CD is its soundtracks. The Japanese and European soundtrack, composed by Naofumi Hataya and Masafumi Ogata, takes inspiration from the contemporary House Music and New Jack Swing genres, and makes heavy usage of sampling as a result. This game also featured the Sonic series' first vocal songs, those being "Sonic ~ You Can Do Anything" and "Cosmic Eternity ~ Believe in Yourself" performed by Keiko Utoku and Casey Rankin.

Due to Sega of America believing that the game needed a "more musically rich and complex" soundtrack, the North American version of the game had an entirely different soundtrack from the Japanese and European versions, composed by the Sega Technical Institute's in-house musicians. Led by Ecco the Dolphin composer Spencer Nilsen, the American soundtrack is more atmospheric and moody than its overseas counterpart, and includes a new vocal theme, "Sonic Boom", performed by Pastiche.

Sonic CD was released in Japan with the original soundtrack on September 23, 1993, then in Europe on October 7, 1993. North America later got the game with the redone soundtrack on November 19, 1993.

Sonic CD was the best-selling title for the Sega CD globally. It sold 1.5 million copies worldwide, powered by 1 million copies sold in the United States alone. For comparison, the Sega CD itself only sold 2.24 million units worldwide.

The original version of the game was ported to the PC in 1996, and was later included in the Sonic Gems Collection compilation in 2005 for the PlayStation 2 and Nintendo GameCube. It was excluded from Sonic compilations for several years due to Sega losing the documentation for the Sega CD — the Gems Collection port is based on the PC version.

To address this issue, the game was remade from scratch in 2011, using the Retro Engine developed by Promoted Fanboy Christian "The Taxman" Whitehead. It was released on Xbox 360 through Xbox Live, PlayStation 3 through the PlayStation Network, PC through Steam, Android and iOS devices, and Windows Phone. ^Phew! The phone version was announced with the others but not released until November 2012 alongside Windows Phone 8.

The new engine allowed CD to be given true widescreen support, and all aspects of the game run at 60 FPS, including the Special Stages. Both soundtracks are available to be swapped between, with this being the first time the Japanese and European soundtrack was officially available in America (other than its inclusion in Sonic the Screensaver, a late-'90s PC app). Further new content was also added, such as achievements and Tails as a playable character.

In May 2021, Sega announced that Sonic CD and the other mainline Genesis Sonic games would be compiled together in Sonic Origins, which was released on June 23, 2022. This version, along with all of the other games in the compilation, runs on the Retro Engine. An update to Origins, Sonic Origins Plus, was announced in March 2023. It was released on June 23, 2023 as both a physical release and as an expansion pack for current Origins owners. This update adds Knuckles and Amy as playable characters. The original Sega CD version of the game is also one of 50 games included in the Mega Drive Mini 2 microconsole, released in October 2022.

Sonic CD contains examples of the following tropes:

• Acid-Trip Dimension: Collision Chaos. The background has, in addition to mountains and a river, a "forest" of yellow crystals and a bunch of yellow circular dots (what the latter is meant to represent is anyone's guess). The foreground isn't much less bizarre, with these things that look like giant frog eggs but break when Sonic hits them, and make a "glass breaking" sound effect. There's also a bunch of panels which display flashing neon symbols in some kind of alien alphabet, and the ground is pink for no apparent reason, which is especially weird considering it's a normal earth/dirt color in the past.
• Action Bomb: Bigbom, a larger version of the Bomb robot from the first game.
• Adaptational Early Appearance: Zig-zagged in regards to Tails being playable in the 2011 and Sonic Origins releases; the game was released after Sonic 2, which doesn't make this a straight example, but with Sonic Origins placing this game between Sonic 1 and 2, it could count as this timeline wise.
  • A straighter example of this is Knuckles being playable in the Sonic Origins Plus release; he originally appeared in Sonic 3, which came after this game both in release order and in timeline order.
• Advancing Wall of Doom: During the Metal Sonic boss race in Stardust Speedway Zone 3, Eggman will chase after Sonic shooting a laser from the bottom of his pod. If he hits Sonic, or if Sonic loses the race, it's an instant kill. The pod itself is also invincible.
• Airplane Arms: When Sonic reaches maximum running speed, he holds his arms out behind him. Additionally, the motion of his legs blurs into a distinctive Mobius strip shape.
• All There in the Manual: With the exception of Metal Sonic and Palmtree Panic's boss, EGG-HVC-001, the bosses originally lacked names^note one piece of concept art shown in Sonic Generations, however, includes an early design of the final boss with the label "Psyco-Egg". Sonic the Hedgehog Encyclo-speedia rectified this in 2021.
• Alliterative Name:
  • The name of every Round features two words that both begin with the same letter: Palmtree Panic, Collision Chaos, Tidal Tempest, and so on...
  • The final boss music has the alliterative name of "Final Fever" in both soundstracks, with the Japanese version having a unique track and the American version having a remix of the regular boss theme due to time constraints preventing the composition of a unique track for the latter version.
• American Kirby Is Hardcore:
  • The North American version replaced the game's house, techno-rave, and new jack swing-influenced soundtrack with a moodier, atmospheric rock soundtrack by Spencer Nilsen of the Sega Technical Institute, known for his work on Ecco the Dolphin.
  • The cover art inverts this: The North American Sega CD and PC versions have Sonic snatch a Time Stone just out of Metal Sonic's reach, while the Japanese Mega-CD cover has Sonic and Metal Sonic moments away from coming to blows◊.
• Amusement Park: Stardust Speedway becomes a massive amusement park in the Good Future.
• Ant Assault: Antons are one-wheeled mass-produced Badniks modelled after ants who tend to serve as basic enemies, driving back and forth and dealing Collision Damage if Sonic bumps into them.
• Anti-Frustration Features:
  • Due to the sheer complexity of the Time Travel sidequest, the game has a save feature that autosaves after completing a world.
  • If you execute a time warp while the timer is at more than 5 minutes out of 10, the timer will reset back to the 5'00"00 mark. This feature is missing in the 2011 remaster, however.
  • Many zones have at least one Past sign located close to some sort of stage gimmick that allows for easy time-travel. The most common is simply two springs facing one another, ricocheting you back and forth until you time-travel, though most stages have their own specific gimmicks that allow for this.
  • While time travel is normally pretty difficult, due to having to run at full speed for around 30 seconds without stopping or running into anything, there are a few locations with two springs facing opposite directions, allowing Sonic to just bounce back and forth until the time travel activates. There are a few other level gimmicks that similarly allow easy time travel, such as these weird spinning things in the Wacky Workbench zone. All you have to do is hit a time travel post, then jump inside.
  • If the timer is running low in a Special Stage, the game will automatically spawn an extra UFO that adds 30 extra seconds to the timer. And if you destroy it, but time runs low again, it will respawn and let you destroy it again for another time boost. As an extra mercy, you don't have to destroy this extra UFO to win the Time Stone either.
  • Getting all seven Time Stones automatically grants you a Good Future in each Zone and the Good Ending, meaning a player doesn't have to waste extra time trying to find the remaining robot teleporters, which will already be destroyed.
  • Destroying the robot teleporters in the past will automatically destroy all of the Badniks in that zone.^note Although any surviving Badniks will still be there if you return to the present.
  • Charging up a Spin Dash or Super Peel-Out will move the screen ahead of Sonic to give him a clear view of what's up ahead.
  • If a player fouls up the time travel sidequest in a zone or runs into a rare Unwinnable situation (i.e. getting stuck in scenery from time travel, the accidentally unbeatable Past Metallic Madness in the game's original release), pausing the game and pressing any button will restart the current zone, albeit at the cost of an extra life.
  • If Sonic is still above solid ground when he tries to jump off the screen if you leave him idle for three minutes, you can resume playing as him without getting an instant Game Over. Made more obvious in the mobile version of the 2011 remake, which had the buttons disappear as soon as Sonic jumps.
  • In the Sonic Origins version of the game, restarting a level outside of Act 3 will have Sonic or Tails remain in their current time zone, which is useful if either miss any machines in the past or need another go at time-travelling if the time posts have already been used. Any already-destroyed machines will also remain destroyed if you do this.
  • In Anniversary Mode of the same game, the player can also use coins to retry a Special Stage if they fail on certain attempts.
• Anti Idling: If Sonic is left idle for three minutes, he declares "I'm outta here!" and jumps off the screen, causing an immediate game over regardless of remaining lives.
• Arc Welding: In addition to having been made as a direct sequel to Sonic the Hedgehog, one officially licensed source from 1994 states that Sonic CD takes place between the events of Sonic 1 and 2. Though the official stance on this was uncertain for some time, Sonic Origins also places it between the first and second game.^note It's sometimes believed that this was retconned to somewhere between Sonic 3 & Knuckles and Sonic the Hedgehog 4 instead, but this is not true; similarly, designer Yasushi Yamaguchi's statements in a 1993 interview give the impression that the game is meant to chronologically take place between Sonic 1 and 2 after all, but his exact phrasing is vague.
• Art Shift:
  • The opening and ending cutscenes, besides switching from sprite-based gameplay to full-fledged animation, also show Sonic with a noticeable variation on his typical design — he's more angular with pointier quills.
  • Unlike the CD-based Present and Future music, the music for the Past levels is sequenced PCM audio to emulate the "past" style of 16-bit music. As a result of sequenced music not being as easily replaceable as CD audio, the Past themes remain unchanged in the US soundtrack, which can lead to some Mood Whiplash at times (such as the track of Metallic Madness Past having a more dire-yet-frenetic feel in contrast with the more mechanical but slower-paced feel of its Present counterpart's US track).
• Artifact Title: The 2011 remake is not distributed on a physical medium, let alone a CD.
• Artistic License – Physics: Everything about the chained-up Little Planet has this in spades. Even considering how close Little Planet seems to be, the chain would have to be hundreds of miles long. Also, even if Little Planet is really tiny (like the size of Phobos or Deimos, which seems unlikely as it has Earth-like gravity and atmosphere) having it that close to the main planet should realistically cause some sort of tidal waves and the like. Also, there's no real reason for it to float away like a balloon when the chain is broken, as the planet is the closest source of gravity it would be more likely to just stay in orbit. To say nothing of chaining a planet to another planet in the first place.
• Awesome, but Impractical: The Super Peel-Out is incredibly fast but it leaves Sonic vulnerable to attacks, making it easy to run into an enemy you can't see coming from offscreen. This is made worse in the 2011 remake, which unlike the original, doesn't move the camera ahead when Sonic is running or charging the Super Peel-Out if the Spin Dash is set to "Genesis" mode. This can be averted by immediately rolling after releasing, thus acting like a much faster Spin Dash.
• Background Music Override: In the Japanese/European Mega-CD version, as well as the 2011 version, the race with Metal Sonic uses the Bad Future theme even if you've achieved a Good Future for the Zone. This is not the case in the North American Sega CD version or any regional release of the PC or Gems Collection versions.
• Bad Future: The Trope Namer. If you complete a level without fixing Eggman's screwing with the past, the result will be a Techno Dystopia with broken and rusted machinery everywhere.
• Band Land: Stardust Speedway's level track aesthetically resembles a gigantic brass instrument — complete with valves and bells.
• Batman Cold Open: The opening animation sequence by Toei demonstrates Sonic's speed, agility, and power in excellent form. Also included in the package is an ending sequence that's equally action-packed.
• Bonus Stage Collectables: The Time Stones are only collectible through the Special Stages.
• Boss-Only Level: The third zone of each round, much like the 8-bit Sonic games, only features a short platforming section before the boss fight. Collision Chaos and Stardust Speedway are close to embodying this completely, as most of the former is a pinball arena with no enemies to speak of, and the latter focuses on a race against Metal Sonic.
• Bottomless Pits: Unlike just about every other Sonic game ever, Sonic CD is almost devoid of bottomless pits. Almost:
  • Changing time periods in an unexpected place in Palmtree Panic will sometimes cause you to de-warp in the middle of scenery. Usually, a little movement and jumping will cause you to glitch through the walls back into the stage proper, as the programmers clearly expected this to happen. On rare occasions, however, it will drop you below the stage, killing you.
  • One of the obstacles in Metallic Madness Zone 3 is a large bottomless pit with some tricky platforming above it.
  • There's one in the Present stage of Stardust Speedway Zone 1 right before the end of the level. However, it's almost impossible to get down there and even if you do, there is still a way back up. You have to deliberately kill yourself to fall into that pit. There's also one in the Past of Stardust Speedway Zone 2. Like the example mentioned above, there is a short wall and spring blocking the drop, meaning you have to try to fall down there.
• Break Them by Talking: The Japanese soundtrack's version of Metallic Madness' Bad Future attempts to do this to the player by featuring a robotic voice saying some demotivational words:
  "You can't do anything, so don't even try. Get some help. Don't do what Sonic does. ...Sonic, dead or alive, is m-m-mine!"^note This is an inversion of a line from the theme song "You Can Do Anything": "...'cause if you try, you can do anything!"
• Breaking the Fourth Wall: As usual, Sonic gets impatient if he's idle. Stand still for three minutes without pausing, and then he'll say "I'm outer here" [sic] and proceed to jump off the screen, netting you a Game Over regardless of how many lives you have left. The exception is in Palmtree Panic Zone 1, where he won't do this if Amy is hugging him down, though he still shoots an angry glare in his idle animation. In the 2011 remake, Tails yawns at you as usual. However, he won't force the game to end after three minutes of idling.
• Bubble Shield: Robotnik in Tidal Tempest zone would go down in a single hit if not for the ring of air bubbles surrounding his eggmobile. Fridge Logic kicks in when you realize the boss fight takes place underwater, and it'd probably easier to just camp above where Sonic can't reach him instead of engaging him with an easily destroyable shield made of one of the only things that can circumvent his inability to swim.
• Bubblegloop Swamp: Quartz Quadrant in the past. The present is instead an Eternal Engine set underground.
• The Cameo: Tails shows up with the Tornado in the D.A. Garden and with a buggy in a secret illustration captioned with "See you next game!" The buggy itself is a cameo of a Caterham 7, his preferred Cool Car model first seen in a December 1992 CoroCoro Comic manga. For the 2011 remake, he is Promoted to Playable.
• Canon Immigrant: Amy^note brunette and sans Rose first appeared in an April 1992 Shogakukan manga.
• Cap: Curiously, Sonic CD's speed cap seems to be based on the one used for Sonic 2 instead of the original game. Pressing forward on the ground maintains Sonic's speed, but pressing it while jumping or falling will trigger the speed cap.
• Chase Scene:
  • If you choose to gather the Time Stones to achieve the Golden Ending, each Special Stage requires Sonic to chase down and destroy a number of flying saucers.
  • Stardust Speedway Zone 3 is Sonic racing Metal Sonic.
• Chicken Walker: The first boss, EGG-HVC-001, has backward facing knees, with spiked feet and springs for fists that knock you backwards.
• Climax Boss: Metal Sonic. Prior to this point, Metal kidnapped Amy Rose and changed the past to help Dr. Eggman rule the future, leaving Sonic to spend the entire game undoing his actions. When Sonic finally confronts him, they duke it out in a style of Boss Battle previously unseen in this series, racing, along with appropriate music.^note The former plays in the Japanese and European versions, the latter in the North American version. This race concludes with Sonic finally rescuing Amy and defeating Metal, with only Eggman left to oppose him. It's the most memorable moment of the game, as well as the most challenging.
• Comic-Book Adaptation: Issues 26-28 of Sonic the Comic and issue 25 of the Archie comics adapt the events of this game.
• Cosmetic Award: Destroying the Metal Sonic holograms in the Past has no effect on the futures or the ending, but it does make some animals appear. The remake gives you a little more incentive to destroy them by tying it into an achievement/trophy.
• Crapsack World: All of the Bad Futures are this, due to being highly polluted dystopias where everything is in a state of severe disrepair. The worst are probably Palmtree Panic (all the trees are dead and the grass is a weird purple color) and Tidal Tempest (where the water has been replaced with some kind of toxic pinkish fluid).
• Creator Cameo: Composer Naofumi Hataya has a small cameo in the JP soundtrack. He's the guy screaming in the background of Quartz Quadrant's Bad Future music.^note The soundtrack on iTunes mentions this.
• Creepy Children Singing: They appear in the Japanese version of the Palmtree Panic Bad Future theme, which is best described as the Evil Twin of the Palmtree Panic Present theme. It takes the cheering children from the original song and distorts them to the point of barely being recognizable.
• Cutting Off the Branches: Sonic Generations and Sonic the Hedgehog 4: Episode Metal heavily implies that the Metal Sonic race canonically took place in Stardust Speedway's Bad Future, though due to the Timey-Wimey Ball, even this may not strictly be true.
• Cyberpunk: As in the first game, Sonic is a Nature Hero fighting against an Evil Genius who is systematically turning the world's inhabitants into robots. The cyberpunk elements are actually expanded in this game, with nearly every zone demonstrating that Little Planet is full of advanced technology. Notably, Sonic isn't there to destroy the technology: he's there to prevent Eggman's Assimilation Plot.
• Cyberpunk Is Techno: The JP soundtrack gives the present time period of the technologically advanced Wacky Workbench a fast-paced and chaotic techno tune. The US soundtrack gives all time zones of of Metallic Madness a harder-driving techno rock sound.
• Damsel in Distress: Amy's entire purpose in this game is to be kidnapped by Metal Sonic.
• Darker and Edgier:
  • Sonic CD is pretty dark in comparison to most of the other Genesis Sonic titles due to the presence of the Bad Futures. Unlike the previous games, where the difference between the "bad" and "good" endings is pretty minimal, here Sonic's work will get retconned by Eggman if he fails to get all the Time Stones.
  • The game over and boss themes of the North American soundtrack are much more dissonant and ominous compared to the Japanese version.
• Defiant Stone Throw: In the bad ending, Eggman is forced to flee his exploding fortress with a Time Stone in hand. Sonic performs an epic, Spin Dash-boosted rock toss at him as he flies away, blowing him up entirely. The only problem is, Eggman simply retcons time so that he never lost.
• Demoted to Extra: In the original version of the game, Tails only makes a couple of cameo appearances in the D.A. Garden and one of the hidden sound test images. In the remake, he's an unlockable character.
• Developer's Foresight:
  • To ensure that players can't sneak in an extra hit point that breaks the game's scripting, Eggman is specifically coded to be invulnerable after the fourth hit he takes in Tidal Tempest until all of the bubbles he summons finish spawning. Even if by some miracle you do hit him again despite his massive bursts of speed, it won't affect him.
  • The final boss, the Egg Spinner, is likewise coded to be invulnerable during specific periods (such as when he's out of the player's reach) so a player can't use an exploit or cheat (i.e. Debug mode) to bulldoze through him right away.
• Developer's Room: One of the cheat codes brings up a screen featuring the developers' time attack records.
• Downer Ending: In The Stinger of the bad ending, there's a time rift, and Little Planet reappears, covered in its metal casing and chained once more, implying Eggman used the Time Stones to hit the Reset Button on his failure.
• Dub-Induced Plot Hole: The prologue from the North American manual tells a wholly different version of Sonic's arrival at Never Lake, with him being accompanied by Amy, and the latter getting kidnapped. By contrast, the opening FMV shows Sonic arriving alone, and Amy isn't kidnapped until the beginning of Collision Chaos. The North American manual prologue also shows Sonic being more friendly toward Amy (as she acts less like a Stalker with a Crush), which is at odds with his demeanor toward her in the ending FMV. This is because North American version (before being revised for the 1996 PC version) renames Amy to Princess Sally, presumably as an attempt to tie in to the cartoon, despite Sally looking and acting completely different from Amy.
• Dub Name Change: The North American Sega CD version renames Amy Rose to Princess Sally. All subsequent games and re-releases of Sonic CD reverted her name back to Amy. Also, the name change appears in the North American manual only; even the English storyline in the European manual still uses the name "Amy".
• Early-Installment Weirdness:
  • Amy makes her first appearance here, in a different dress, with different hair, and lacking her trademark hammer.
  • Zigzagged with the game as a whole. As in Sonic 1, there are three acts in every zone instead of two, Special Stages are accessed by collecting 50 rings and jumping into a Giant Ring at the end of the level, and levels are more focused on platforming and exploration instead of speed, which is compounded by the game's time travel mechanic. However, this game came out after Sonic 2 which had two acts per zone, had levels focus on speed instead of platforming, and introduced a different method of accessing Special Stages. Then Sonic 3 came out after this game and built on the foundation that Sonic 2 laid, ignoring Sonic CD completely. This is somewhat justified as while series creator Yuji Naka and several other key developers split off from the original development team to work on the rest of the main Sonic trilogy in the U.S., what was left of the original development team stayed behind and worked on Sonic CD, while Naka's team worked on Sonic 2 simultaneously.
  • The Spin Dash is notably different from the one in Sonic 2 and on, as it has a different spinning animation and has to be charged by simply holding it for a second instead of button mashing it.
  • Even the 2011 remaster, when compared to the 2013 remasters of Sonic 1 and 2, has shades of this. The process to go from the title screen to the game is different^note Sonic 1 and 2 have a separate remake title screen and file select, while Sonic CD has its file select incorporated into the game itself, there is no option to play as Knuckles or Sonic and Tails together^note though Knuckles was added as a playable character in the Origins Plus version, the mobile version has a different button and D-pad design, the game is a multiplatform release (Sonic 1 and 2 were mobile-only), and it was a solo project by Christian Whitehead. Unlike later ports, Stealth had no involvement at all, which could explain various differences such as Knuckles not being available.^note Stealth was the person responsible for making the Knuckles in Sonic 1 hack.
• Easter Egg:
  • A statue in Wacky Workbench Zone 1,^note It's either an angel statue in the past that gives rings, or an Eggman statue that drops bombs in the Bad Future; finding the angel statue unlocks an achievement in the 2011 re-release, while in Sonic Origins Plus, destroying the Eggman statue with either Amy or Knuckles will fill in part of the compilation's special illustration. some artwork, and a dummied out Special Stage are all hidden in the game.
  • The secret Sound Test in the original release contains a number of these. Entering the right combination of three (two in the PC / Gems Collection version) numbers will take you to several pieces of drawings, start a secret Special Stage, or unlock Debug Mode.
• Eternal Engine: Metallic Madness and Wacky Workbench, as well as most of the Bad Future levels being combined with the current level trope. Wacky Workbench Good Future tones this down slightly, being more focused on toy imagery. Tidal Tempest is this in both futures, being a run down base in the Bad Future, while being an aquarium of some sort in the Good Future.
• Ethereal Choir:
  • The Japanese final boss music has a woman singing in the background; there are no lyrics, so it's essentially a One-Woman Wail with instruments.
  • The Bad Future version of Tidal Tempest in the U.S. soundtrack has a chorus chant.
• Evil Knockoff: Metal Sonic, a robotic doppelgänger of Sonic himself, makes his debut here.
• Evil Is Not Well-Lit: All of the Bad Futures are much darker than any other time period, be it due to pollution darkening the sky, artificial light sources long having burned out, etc. They manage to pull this off even with Collision Chaos Bad Future, as while it's still pretty colorful, the sky is both dark red and permanently overcast.
• Fake Difficulty:
  • A lot of the difficulty in the game comes from the fact that the more exploration-based level design, which has lots of cheap obstacle and enemy placement, simply isn't made with Sonic's speed or sensitive controls in mind (or barring that was deliberately made to work against his abilities) and thus making it very easy to accidentally slam directly into an enemy or obstacle if you try to maintain a consistent speed more often than not - Trial-and-Error Gameplay is in full effect here. Metallic Madness is particularly nasty in this regard, with some enemies that are placed in near-unavoidable spots or edgeguarding a path you need to traverse. Not to mention you only have a very limited amount of chances to time travel, and it's very easy to bungle it if you so much as hit a road bump, forcing a player to reset or quit if they're aiming for the good ending. On top of that, the teleporters in the past tend to be well hidden—the one in Wacky Workbench Zone 1 is especially aggravating, as it requires a logic-defying way of reaching it. The original version of the game even had a flat-out unwinnable situation in Past Metallic Madness if you used up all of the Future signs. This was fixed in the 2011 remake.
  • The Special Stages have this in the form of the UFOs' bizarre hit detection (their hitbox is smaller than their sprites) combined with the high speed jumping and confusing perspective.
• Fake Trap: Wacky Workbench Zone 1 contains fake crusher traps. One of them is required to find the robot transporter in this stage.
• Fan Remake: Similarly to the Retro Engine remasters of Sonic 1 and 2, Sonic CD also received a fan decompilation port that allows the Steam and mobile versions of the game to be played natively on PC and other platforms while providing unique benefits from both versions that one version didn't receive from the other.^* The Steam version of Sonic CD didn't receive script updates that the mobile version has gotten, while mobile version has a different rendering backend that doesn't fully support palettes compared the Steam version. The fan decompilation also has native support for user-created mods.
• Flower Motifs: The Badniks contain seeds which sprout into flowers, as opposed to the "animal battery" motif the series normally uses.
• Foreign Re-Score: The North American version replaces four-fifths of the soundtrack by Naofumi Hataya et. al with new music composed by Spencer Nilsen and David Young.^note The "Past" music for each Zone is exactly the same in both the US and JP versions –- probably due to technical issues, see "Painting the Medium" below.
• Game-Breaking Bug:
  • It's entirely possible to get Sonic stuck within scenery by accident depending on where you were/are at once you time travel, which softlocks the game and forces the player to reset. Naoto Oshima was apparently aware of this issue and his development team at least tried to accommodate the level design to prevent this from happening as much as possible, so it usually takes some effort or sheer bad luck on the player's part to pull this off. invoked
  • Move Sonic to behind the starting point (i.e. left of "0000") while using Debug Mode, and the game will crash.
  • There's also a bug which stops the music from looping and the next level from loading should the player take too long.
  • There's a point in the Past of Metallic Madness where you have to navigate some springs. The problem is that in the original versions of the game, you cannot reach the top platform. The only way around it is to try to get back to the present. If you can't, you are stuck. Extra platforms were added in the 2011 release to remedy this issue.
• Game Mod:
  • The game has received relatively few hacks compared to other games in the classic Sonic series due to the complexity of the Sega CD, though a few folks have taken cracks at it. Sonic CD Plus Plus is one of them, adding new features like the Homing Attack and a Sonic 2 style Spin Dash to the game.
  • The Steam version and fan decompilation port has received various mods thanks to the Sonic CD Mod Loader and the decompilation port's native modding support, respectively, such as making Amy Rose playable, restoring the unused Desert Dazzle Zone, and implementing the original time travel mechanics into the game. There's also Sonic CD Restored, a mod that aims to restore many elements from the Sega CD version into the Steam version and the fan decompilation while adding some quality-of-life improvements, the original time travel mechanics, and as part of a 1-Year Anniversary update, added Amy as an unlockable character, all while remaining faithful the original Sega CD version's features. There are also add-ons for Sonic CD Restored that can add Knuckles as an unlockable character and customize the game's US and Japanese soundtracks.
  • One mod for the Steam release adds newly-created Past tracks for the North American soundtrack, addressing the dissonance that occasionally occurs otherwise due to the original US composers being unable to replace the sequenced tracks.
• The Glomp: Amy will try to do this to Sonic twice: once in Palmtree Panic, and again in Stardust Speedway after he frees her.
• Golden Ending: The good ending involves the final cutscene showing the Little Planet being freed, and a stinger where sparkles from the liberation of the Little Planet cause flowers to bloom underneath. It's achievable one of two ways:
  1. Get all seven Time Stones.
  2. Destroy all the robot transporters in the past levels.
• Gotta Kill Them All: You can get the good ending without beating any Special Stages by going to the Past in every Zone and destroying a unique machine, which also destroys all robots in the past. Doing so also lets you go to the Good Future in the same level. Additionally, the Special Stages themselves require you to kill all the flying saucers in the stage to get the Time Stone.
• Green Aesop: Just like the previous game, the main threat from Eggman is the damage he intends to do to nature. Notably, technology itself is not the problem: the problem is Eggman. In the Bad Futures, which have Eggman take over the Zone, the environment is in a severe state of disrepair - including the robots themselves (showcasing Eggman's wasteful ways). In the Good Futures, it's shown that nature and technology can and indeed should be in the same place to create paradise. For example:
  • Stardust Speedway. In the past, it was a vine-covered Temple of Doom sitting just outside of a massive city that seemed to have a flair for art and theater. In the present, it's developing into an advanced city, and the level track now resembles a gigantic brass instrument. In the good future, the city has become a massive amusement park. In the Bad Future, the city is ludicrously dense with factory buildings that fill up quite literally all available space to the horizon, filling the once beautiful, starry sky with dense smog and occasionally lightning. All signs of art and culture have been destroyed.
  • The Good Future of Palmtree Panic is inhabited by small squirrels and has a system of pipes that regulate the clear water in the area, instead of spreading polluted water, which happens in the Bad Future. The Good Future of Tidal Tempest looks like a giant hydroponic greenhouse with nutrient-rich green water; in the Bad Future, the plants are dead or roboticized, and the water is magenta.
• Guide Dang It!: For players trying to destroy all the robot transporters in the past, Wacky Workbench Zone 1's device is infamously difficult to reach on the first try. In order to reach the machine, you have to jump on top of a crushing piston near the level's end, which sends you down the pipe where the transporter is located instead of killing you as with most of the crushing pistons in the series. Fortunately, it's possible, albeit difficult without the skill and/or Save Scumming, to have all the Time Stones by this point for those who don't want to bother with the transporter.
• Hailfire Peaks: This trope is common in this game, though it often only applies to the visual design instead of the gameplay:
  • Palmtree Panic is this game's Green Hill Zone, but has Eternal Engine elements in both the Good and Bad Futures.
  • Collision Chaos is Green Hill Zone or The Lost Woods combined with Pinball Zone.
  • Tidal Tempest is Underwater Ruins in the Present, Underground Level and Under the Sea in the Past, Underwater Base and Techno Wreckage in the Bad Future, and more of the same in the Good Future, only it's more of an aquarium.
  • Quartz Quadrant is Underground Level, with elements of Eternal Engine aside from the Past, which is instead Bubblegloop Swamp. The Good Future seems to feature an Underground City in the background.
  • Wacky Workbench is full time Eternal Engine, but combined with Toy Time in the Good Future and Construction Zone Calamity in the Past.
  • Stardust Speedway is mostly Skyscraper City, but features an Amusement Park in the Good Future and could be implied to be Wretched Hive or City Noir in the Bad Future.
  • Metallic Madness is almost entirely Eternal Engine, but with a few Green Hill Zone elements such as green grass in the Good Future. Even in the Good Future, it's still The Very Definitely Final Dungeon.
  • All the Bad Future stages feature the current trope combined with Bleak Level.
• Heroic Mime: Subverted. Sonic will let out a "Yes!" when getting an extra life, as well as saying "I'm outta here." when being idle for too long. Played straight in the Origins version, which removed all the voice clips.
• Hitbox Dissonance: In the 2011 remake, the sawblade hitboxes on the rhino beetle badniks seen in Metallic Madness are mismatched with their animation. This was never fixed, even in the Origins compilation.
• Inconveniently-Placed Conveyor Belt: These are everywhere in Quartz Quadrant. In the past, they move fairly slowly, in the present they are average speed, and in either of the future stages, they move pretty damn fast, so getting past them can be a bit annoying. You can change the direction they go in, but it doesn't always help. This culminates in the Egg Conveyer boss.
• Impact Silhouette: In Palmtree Panic Zone 1 Present and Past, if Sonic enters a tunnel in the middle of the level, he will emerge by comically smashing through the terrain, leaving his silhouette.
• Improbable Aiming Skills: During the bad ending, just look at the trajectory that Sonic's rock makes as he throws it at Eggman.
• Indecipherable Lyrics:
  • The JP Metallic Madness Present theme has a verse which is pretty much unintelligible. The only parts that are understandable are "Get yourself ready for a fight" and what sounds like "Yo, we're outta here!"
  • Also from the JP soundtrack, the boss music can be hard to understand aside from the "Work that sucker to death! Come on now! Work that sucker to death!" chorus and the spoken part that describes the foregone conclusion of the boss himself not standing a chance.
• Insistent Terminology:
  • Whoever is behind the terminology in the series decided that what most Sonic games call an Act, this game would call a Zone.
  • What are usually Zones in Sonic games are called "Rounds" here. "Act" is nowhere to be found anywhere.
• Instant-Win Condition: Normally, to get the good ending in Sonic CD, you have to go to the Past to find and destroy the Robot Generator in the first two Zones of every single Round. Missing even a single generator hardlocks you into the Bad Ending. However, should you get all seven Time Stones before the end, you automatically get the good ending even if you missed every single generator beforehand. Even better, any and all remaining levels you go through before the end will automatically show a Good Future whenever you go there. And if you're skilled, the earliest you can achieve this is the end of Quartz Quadrant Zone 1; which is roughly halfway through the game.
• Interquel: Sonic CD takes place between the original Sonic the Hedgehog and Sonic the Hedgehog 2. Originally, this was only indirectly implied by the game's designer, Yasushi Yamaguchi, but it was later reiterated as canon by the Sonic the Hedgehog Encyclo-speed-ia and Sonic Origins. This is why Tails (introduced in Sonic 2) only makes a hidden cameo appearance, and why Super Sonic doesn't appear — though that's more due to the main collectibles being Time Stones rather than Chaos Emeralds.
• Ironic Echo: Doubling as a Dark Reprise: the Japanese music for Metallic Madness Bad Future features a robotic voice (presumably Eggman himself) mocking the player with an ironic inversion of the intro music's lyrics.
  Intro: [...]'Cause if you try, you can do anything!
  Metallic Madness: YOU CAN'T DO ANYTHING, SO DON'T EVEN TRY.
• The Juggernaut: Metal Sonic is completely invincible to Sonic's attacks thanks to an electrified force field he has, and his boost function allows him to effortlessly smash through obstacles Sonic has to work his way around. Sonic is only able to defeat him by using a combo of the closing goal gate of the race course they're on, with Eggman's instant kill laser finishing the job.
• Leitmotif: The JP boss music for Eggman, due to part of the lyrics:
  The war began. Earth trembled in fear. Bald, brainy, and cold-hearted. The distance grows closer, little by little. Wow! The power you possess is incredible! He doesn't stand a chance!
• Lighter and Softer: The North American track for Wacky Workbench Bad Future over the Japanese/European track. This definitely seems to be averted for Collision Chaos and the boss and game over themes, however.
• Loophole Abuse: Despite having a ten minute limit per act, it's possible to spend far more time than that in any one level. A successful time warp will revert the stage timer back down to five minutes. If you've been playing for less than five minutes, the timer will be unaffected. Also, if you're in Debug Mode, the timer doesn't seem to work at all. Just don't go too far to the left. This is averted in the 2011 remaster, which is missing this feature.
• The Lost Woods: Collision Chaos is a forest with a lot of mechanical elements thrown in.
• Luck-Based Mission: Tidal Tempest's Egg Bubble boss is a One-Hit-Point Wonder, yet has the ability to suck up a bunch of air bubbles and arrange them in a sort of rotating "shield" that Sonic has to breathe away in order to hit him. The boss will periodically re-arrange the shield to cover up the gaps. Sometimes you'll get lucky and only have to breathe away one or two before you manage to hit him, while other times you'll have to breathe up almost all of them before you can get the hit in. Furthermore, it's possible to breathe them all up, and then die if you don't deliver the hit soon enough as you no longer have any air supply.
• Macro Zone: In some of the end of Metallic Madness Zone 2, Sonic goes through shrink rays that make the zone fit this trope. He still retains normal jump height though.
• Missing Secret: For those who aim for 100% completion, Metallic Madness can be puzzling at first when it comes to finding Metal Sonic holograms. In fact, it doesn't have any. This makes sense since he was defeated in the previous Zone, albeit in the future.
• Mocking Sing-Song: The Japanese game over theme samples the intro song to enforce this trope.
• Mook Promotion: The Hotaru Badnik from Stardust Speedway later shows up as the mid-boss before the final boss.
• Mosquito Miscreants: The Needlenose enemy is a robotic mosquito that plunges down at Sonic if he gets near him.
• Mythology Gag:
  • The screen that displays when unlocking Tails in the 2011 remake reads "SEE YOU NEXT GAME," a reference to Tails' cameo in the original game (which is still accessible here).
  • The Time Attack menu theme in the 2011 remake, with the Japanese/European soundtrack selected, is an instrumental version of "Brand-new World", a track from the Sonic the Hedgehog - Remix album released in 1994.
• New Jack Swing: This game, alongside Sonic 3 & Knuckles, is one of two "Classic" era games to have its soundtrack heavily influenced by the genre. However, up until the release of the 2011 remake, the only versions of the game with that soundtrack was the Japanese and European releases. The only New Jack-styled track from CD to appear in the North American release prior to 2011 was the "Past" mix of Stardust Speedway.
• No Fair Cheating:
  • In the remake, activating Debug Mode disables achievements/trophies, and Zone times won't be recorded.
  • Also in the remake, Tails, as a tradeoff for his flying skills, cannot be used to unlock achievements/trophies, as well. Furthermore, during Wacky Workbench's Egg Razer boss, his flight time period gets shortened significantly, preventing you from Sequence Breaking the boss by going further ahead of Eggman. This also applies to the start and end of the Metal Sonic race in Stardust Speedway.
• No-Nonsense Nemesis: When the Egg Razer in Wacky Workbench is destroyed, the screen is covered by the steam it leaks. When it clears up, Sonic appears to have just attempted to crush Dr. Eggman inside the machine, but the villain escaped by a hair. In the bad ending, Sonic also attempts to blow him out of the sky to keep him from escaping with the Time Stones, but it doesn't work either.
• No Ontological Inertia: Destroying the roboticizer in the past time period of a given level makes all the surviving Mooks instantly explode.
• No OSHA Compliance:
  • Wacky Workbench, a factory where the ground floor bounces things high up into the air at random intervals (or constantly in the Futures).
  • Quartz Quadrant wouldn't fit OSHA standards either with spikes everywhere.
  • Metallic Madness is the most obvious example, being a massive, mazelike, and deadly level laden with buzzsaws, crushing pistons, and tons of different badniks... plus a bottomless pit section in Zone 3.
• Non-Indicative Name: If the name of a Round refers to a background element rather than a gameplay mechanic, chances are it's completely absent in its Bad Future. Palmtree Panic's Bad Future has all of its palm trees replaced with machines, Quartz Quadrant's Bad Future has been mined bare of its quartz, and a thunderstorm hangs permanently over Stardust Speedway's Bad Future, blocking all of the stars.
• Non-Lethal Bottomless Pits: If you fail to platform in the methodical Wacky Workbench Zone 1, instead of dying to a pit the bouncy floor just wastes your time by sending you to the top of the level, which in the present version of the level is a series of trap rooms with no way out but back below.
• Non Standard Game Over: Put the controller down. Three minutes and one "I'm outer here" [sic] later...
• Oddball in the Series: This game feels a little unusual compared to other main 16-bit Sonic games, even disregarding the time travel gimmick and the use of CD-quality audio:
  • Sonic's jump sound is significantly different.
  • There is no sound effect for when Sonic rolls into a ball.
  • While there is a Spin Dash available, it doesn't use the typical Spin Dash animation; Sonic just goes into his rolling animation. This can be explained by the fact that this game's development began before Sonic 2's, so they were still getting the kinks worked out.
  • In addition to the Spin Dash, there is a Super Peel-Out move that grants a similar boost but without sending Sonic into a roll.
  • What other games in the series call "Zones", this game calls "Rounds". "Zone" in this game is used to refer to what other games call an "Act".
  • The time counter is in the format of (minutes) ' (seconds) " (hundredths of a second), rather than (minutes) : (seconds) like in the Genesis games. Although this format would later be reprised in Sonic Mania.
  • Special Stages reward Time Stones rather than the traditional Chaos Emeralds.
  • Said Time Stones are not required for the Golden Ending. Destroying all the robot transporters also works.
  • The speed shoes item, rather than speeding up the music, has its own theme.^note This is likely because speeding up CD audio and having it not sound like a mess was not possible with early '90s consumer hardware.
  • Much of the soundtrack is completely different between regions, whereas in other 16-bit Sonic games the most changes made to one game's soundtrack was replacing a few tracks in the Windows port of Sonic 3.
  • The camera in this game moves ahead to show the level ahead of Sonic, while the other games had Sonic centered at all times.
  • Most of the bosses take a few hits to defeat, unlike the standard eight hits for bosses in other Sonic games. Even in the last fight in Metallic Madness, Eggman has only 4 HP.
  • The familiar Sonic drowning music is absent in this game; the regular level music continues playing as the drowning timer ticks down, while an urgent bubbly sound plays repeatedly.
• Offscreen Moment of Awesome: The final cutscene implies that Amy managed to get through Metallic Madness on her own just to catch up with Sonic.
• Oh, Crap!: During the Quartz Quadrant boss, Eggman will begin panicking when his pod starts to get destroyed from the friction of the conveyor belt. Once it's worn down enough, the doctor flees as the pod explodes.
• Older Than They Look: Concept art◊ for this game puts Eggman as having been born in 1931. This would make him 62 by the time the game came out, although he doesn't really show it that much.
• One-Hit Kill: Eggman's pacing laser in Stardust Speedway Zone 3 is an instant kill even if you have Rings.
• Our Founder: The statues of Eggman at Wacky Workbench and Stardust Speedway (Present and Bad Future), and the image on the mountain that appears in the opening.^note Unfortunately, the close-up of the mountain tends to be cut in the re-releases.
• Oxygenated Underwater Bubbles: Present throughout Tidal Tempest. Eggman uses them as a shield in that level's boss that you can breathe through and attack him for a One-Hit KO.
• Palmtree Panic: The Trope Namer is the first level of the game, and the local Green Hill Zone.
• Painting the Medium: The Present and Future music tracks are in Red Book audio (i.e. standard Compact Disc Digital Audio, or CDDA), while the Past songs are in the lower quality PCM format used by the Sega CD's PCM chip^note technically, Red Book audio is also PCM, specifically Linear Pulse-Code Modulation (LPCM), but is higher quality than what the Sega CD's PCM chip is capable of.
• Pinball Zone: Collision Chaos, combined with some elements of Green Hill Zone and The Lost Woods. This also goes for its boss, the Egg Tilter.
• Platform Hell:
  • Wacky Workbench Zone 1 is unique — and controversial — among Sonic levels for its heavy focus on platforming. There is a straightfoward single path to the goal, but falling off the platforms will cause Sonic to land on a powerful bouncy floor that sends him towards the closed-off top areas. The player must then fall back and try again.
  • In Metallic Madness Zone 3, the player has to overcome a small area full of temporary platforms over the single bottomless pit in the game and time it just right to run under a crusher and into the final boss corridor. You can cheese this challenge by hugging the side of the crusher when it is grounded, causing Sonic to land on the very tip of the floor. However, this won't work in the remake due to its changed physics.
• Player Nudge: To encourage new players to learn about the Time Travel mechanic, Palmtree Panic Zone 1 conveniently places both a Past and Future sign next to each other by the first tunnel, which upon entering will allow the player to build up enough speed to trigger a time warp after launching out of it (provided they miss a platform that's right above, which is a subtle clue that any interruption of speed will cancel the warp). The placement of the signs is also important; grabbing the Future sign first will likely lead to a new player then accidentally (if not purposefully) grabbing the Past sign beside it or vice versa, which teaches the player that each sign can only be used once and that you can only use one or the other's benefits at any time.
• Precocious Crush: A lot of fans forget that Amy was introduced as eight years old — the same age as Tails in Sonic the Hedgehog 2 — even though she and Sonic were on dating terms in her manga of origin. She was aged up to 12 years old in Sonic Adventure, though Sonic is 15 in that game.
• Pre-Rendered Graphics: Every cutscene in the game is a fully-animated 2D cartoon by Studio Junio and Toei Animation.
• Press X to Die: In the North American Sega CD version, pressing one of the jump buttons while the game is paused will restart the level at the cost of one life provided you have at least two lives available. Otherwise, this reverts the player back to the title screen. This also works in the PC version.
• Promoted to Playable:
  • Tails can be unlocked in the 2011 remake by beating the game once as Sonic. He actually makes the game easier due to his ability to fly and swim (taken from Sonic 3), which raises the question of why they didn't have him there from the start for inexperienced players. As a tradeoff for the easier difficulty, you can't earn achievements while using Tails. He's also playable in the Origins remake.
  • Knuckles and Amy are playable in the Origins Plus update of the remake.
• Puzzle Boss: Compared to the rest of the series, there are a lot more bosses that revolve around puzzles; while most of the bosses are of the "hit him until he's dead" variety, all of them except the first (and, eventually, the last, which gets progressively easier to hit as the battle progresses) also require that you figure the "trick" to hurting them, from manipulative platforming to whittling down shields bit by bit.
• Real Men Wear Pink: For no clear reason, the EGG-HVC-001 is colored pink in the Good Future of Palmtree Panic.
• Remixed Level: Every Zone in the game has four versions of itself for each time period, including the two futures separately. Each of these four versions have minor changes to the layout, though the requirement to take alternate paths is rare due to the nature of this game's time traveling.
• Re-Release Soundtrack: The original Japanese soundtrack was kept for the European release, but the North American version replaced it with a whole new soundtrack mixed in QSound (a positional audio algorithm that attempts to mimic surround sound through stereo speakers). However, only the CD audio tracks were replaced — the past tracks, which are generated through the system's sound chip, weren't able to be replaced. This creates a jarring contrast when traveling between the past and the future. When the game was re-released in 2011, that version used the original Japanese soundtrack by default, but included the option to switch to the North American soundtrack.
• Reset Button: In the bad ending, Eggman uses the Time Stone(s) you didn't collect to revert everything you did in the game.
• Robotic Psychopath: Metal Sonic enjoys torturing and killing animals. He was designed that way.
• Sampling:
  • The lyrics "Work that sucker to death! Come on, now! Work that sucker to death!" come from the Xavier song "Work That Sucker to Death". Worthy of note is that although the original song did say "work that sucker to death" often, this particular sample is only the chorus to the JP Sonic CD boss music.
  • The JP soundtrack samples its opening and ending themes during normal gameplay, with the game over theme sampling the former, albeit slowed down and muffled, and the stage clear theme sampling the latter. Interestingly, both are intact in the 2011 remake despite the lyrics being cut from their sources.
• Rubberband AI: The Metal Sonic race relies on this. How Metal Sonic reacts depends on how far Sonic is ahead and him and for how long. If you're skilled enough to keep far ahead of him nonstop (or cheat with mods like Sonic CD Plus Plus), Metal Sonic will go into nonstop overdrive until the very last stretch in order to keep up with Sonic.
• San Dimas Time: With all of the time travel involved, the game still has the 10-minute limit in its stages. Going to the past, or future (Good or Bad), doesn't change this in the slightest.
• Save Scumming: The 2011 version of the game makes a save at the beginning of a Special Stage and doesn't save the results if the game is restarted before the results screen finishes. This makes it easier to get all seven Time Stones early, and doing it early enough would mean Quartz Quadrant Zone 2 and on automatically have Good Futures.
• Save the Princess: In the North American Sega CD manual, Amy was identified as "Princess Sally". This change was undone in all later sources and re-releases.
• Scary Scorpions: The Sasori enemy is a robotic scorpion that inhabits Quartz Quadrant Zone and attacks by shooting lasers from its tail.
• Scenery Gorn: The Bad Futures. Eggman's corruption hasn't been fixed, and the world has become a nightmarish dystopia. It's especially bad for the levels where water can been seen; the water ends up looking horribly polluted and nasty.
• Scenery Porn: The environments are beautiful to look at, possessing a dream-like quality to them. This especially holds true for the Good Futures, a mix of technology and nature in bright flashy colors. The Special Stages also have a variety of very unique background aesthetics with a great sense of place to them.
• Science Is Bad: The Bad Futures, with all the broken machinery throughout them. Even Eggman's Mecha-Mooks and his own base are broken and rusted with neglected ruin.
• Science Is Good: The Good Futures utilize science and technology for purposes beneficial to the environment, having tech coexisting with nature.
• Screw This, I'm Outta Here: If the player doesn't move Sonic for three minutes, Sonic will say "I'm outta here" and ditch the player by jumping off the stage, resulting in a Non Standard Game Over.
• Scripted Event: A rare story instance of this in a classic Sonic game. When you first arrive at Collision Chaos, you find Amy chasing you around and that the tunnel to the rest of the level is blocked off by some spikes (there's a ledge nearby, but that gets you nowhere either). Cue Metal Sonic arriving, smashing through the spikes, kidnapping Amy and flying off. Even if you use debug mode to skip this part, it's impossible to save Amy—Metal Sonic is completely immune to your attacks and will always grab her, fly off and vanish right away.
• Set Right What Once Went Wrong: Sonic's goal is to save the Little Planet's future by ridding it of Eggman's influence. He can achieve this by destroying the robot transporter in the past or collecting every Time Stone.
• Shielded Core Boss: The bosses in this game generally take fewer hits to trash. They make up for this, however, by having better / more devious defenses than usual, or by only being possible to beat in a non-traditional manner.
• Shout-Out:
  • One of the Easter Egg posters is a parody of Batman (1989).
  • DJ Eggman wears a Jamaican headpiece in the "The Fastest DJ 'MC Sonic'" Easter egg and the 2011 remake's Soundtrack Select screen. The same Easter egg reads "Can I kick it?", a reference to the song of the same name by A Tribe Called Quest.
  • Exclusive to the 2011 remake:
    • The achievement for traveling through time at least once is "88 Miles per Hour".
    • The achievement for finishing any level in the Good Future is "Paradise Found".
    • The "King of the Rings" achievement is a reference to an old obscure Sonic song.
• Skyscraper City: Stardust Speedway in the Present. A slightly imposing looking one serves as the backdrop for Special Stage 5 too.
• Sliding Scale of Shiny Versus Gritty: The contrast between the good and bad futures.
• Smashing Hallway Traps of Doom: Real ones are found in Metallic Madness. Earlier, objects that look almost identical to these are found in Wacky Workbench, but they are completely harmless and actually drop you into secret areas if you stand on them as they slam into the ceiling. This is necessary to reach one of the robot transporters.
• Solar Punk: Traveling back to the Past and destroying the robot transporters found in the first two Zones of each Round will prevent the Bad Future where the environment is corrupted into a Polluted Wasteland. This leads to a new timeline where widespread technology works in harmony with nature to create a lush and vibrant Good Future devoid of Badniks.
• Soundtrack Dissonance:
  • If you except the final boss, the North American boss theme is ridiculously ominous for boss battles against a mini-mecha with bumpers for arms, a pinball machine and a bubble shield, especially in the Good Futures.
  • Wacky Workbench's Futures also fall into this; its Good Future has more emphasis on work tools in the background despite the zone being turned into a toy land, while its Bad Future is accompanied with a disco-like theme in the North American release (the Japanese and European versions have something much more appropriately sinister for it).
  • An interesting example regarding the Metal Sonic race in the Good Future of Stardust Speedway: in the Japanese and European versions, the soundtrack switches to the Bad Future music during the race, but in the North American version, the Good Future music continues to play. The 2011 version of the game changes this so that the Bad Future music always plays during the Good Future race, even with the North American soundtrack.
  • If you are used to Japanese tracks, US Good and Bad Futures can be this, as the former ones are more subtle in their celebration and the latter are rather bittersweet, as they give the player the impression that they can still save the day.
• Spiders Are Scary: The Kumo-Kumo enemy is a robotic spider that inhabits Quartz Quadrant Zone. Kumo-Kumo fires sticky webs at the player, slowing them down for a few seconds.
• The Spiny: The Kemusi Badnik is an expy of Caterkiller from the original Sonic the Hedgehog, and Noro-Noro, from the same Round, will also occasionally show spikes on its back.
• Stealth Pun: The reason levels in this game are called Zones instead of Acts? That's because this game is about time travel; they're time zones.^note That is, "Palmtree Panic Time Zone 1", for example.
• Stepping Stones in the Sky:
  • In the intro movie, Sonic uses the falling debris of the Little Planet as platforms to reach it.
  • Palmtree Panic has small hidden platforms in the present hanging in the sky for no apparent reason. In the past, they are visible.
• Super Title 64 Advance: Sonic CD for the Sega CD.
• Suspiciously Similar Substitute:
  • According to Naoto Ohshima, most levels, with the exceptions of Quartz Quadrant and Wacky Workbench, were made as more surreal versions of the original Sonic the Hedgehog levels. Even the level theme order is the same. Sometimes, the similarities are subtle, sometimes not:
    • Palmtree Panic is obviously inspired by the lush visuals of Green Hill. The Tamabboh Badnik looks like a Moto Bug and behaves like a Crabmeat.
    • Collision Chaos has a Spring Yard at the end of both Zones 1 and 2. The second zone also has two goals, just like the Act 2 of its original counterpart.
    • Tidal Tempest's graphics resemble Labyrinth's. The water current gimmick, which sends Sonic spinning underwater is used in the second zone.
    • Stardust Speedway shares its level theme and position with Star Light as the penultimate level. It plays rather differently, though, besides being fast-paced.
    • Metallic Madness' spinning, disappearing and moving platforms and doors are all from Scrap Brain. There's also a Giant Mook version of the Bomb badnik called Bigbom and each Zone has a unique background, just like Scrap Brain.
  • A scrapped zone, Dubious Depths, likely would have stood in for Marble Zone — it went in between Palmtree Panic and Collision Chaos, and apparently had a similar marble ruins theme. However, the concept of a half-underground level has been kept with Quartz Quadrant.
  • Quartz Quadrant and Wacky Workbench strongly resemble Hidden Palace Zone and Cyber City Zone, respectively. These levels were designed in the early stages of Sonic 2's development. However, they didn't make it into the final version (although elements from them did). It's quite possible that this was a conscious move, as Sonic 2's developers had to cut several levels due to a tight deadline.
  • Kama-Kama/Robot Mantis, a Badnik in Collision Chaos, is based on Kama Killer/Slicer from Sonic 2, but with linear instead of homing projectiles.
  • The Time Stones clearly fill in for the Chaos Emeralds.
• Take That!: The name of the first boss is the EGG-HVC-001. HVC-001 was the model number of Nintendo's own Famicom console: the HVC stands for "Home Video Computer". As a bonus stealth insult, the HVC-001 is the easiest boss in the entire game. Fighting it in the Good Future also colors it pink, and this is probably the only explanation for that.
• Tagline: The page quote.
• Team Killer: If you beat Metal Sonic in Stardust Speedway Zone 3, Eggman's laser will catch up to him, killing the doctor's own creation.
• Techno Wreckage: The Bad Futures all feature technologically advanced garbage in huge amounts - Wacky Workbench and Metallic Madness especially.
• Temple of Doom: Stardust Speedway in the past is a Roman/Greek style city.
• Time Travel: A present mechanic of the game, inspired by various books and films, including Back to the Future.
• Toy Time: Wacky Workbench's Good Future is a brightly colored toy factory, in contrast to the bleak, broken-down warehouse seen in the Bad Future.
• Tube Travel: A lot of it in Metallic Madness Zone 2. Tidal Tempest Zone 1 and Wacky Workbench also have a little of it.
• Underground Level:
  • Tidal Tempest is underground and under water in the present.
  • Quartz Quadrant happens in a cave with conveyor belts: while the Past is a simple cave, the Present is a Crystal Landscape, the Good Future is an Underground City and the Bad Future is another Eggman factory.
• Under the Sea: Tidal Tempest is an underwater area in the present and futures.
• Underwater Base: Tidal Tempest in the Bad Future is a bleak underwater area. Special Stage 3 also has one serve as background scenery.
• Underwater Ruins: Tidal Tempest, though oddly not in the past as it's just a partially-submerged cave.
• Unique Enemy: The Taga Taga enemy found in Tidal Tempest also appears in Palmtree Panic, where it jumps out of waterfalls like Chopper from Sonic the Hedgehog, but this version is rare enough that you won't meet it unless you explore.
• Unlockable Content: The Special Stages in Time Attack, the D.A. Garden (a Sound Test that lets you listen to all of the CD-ROM tracks, plus an image of the Little Planet) and the Visual Mode (the Video Gallery).
• Updated Re-release:
  • The initial PC version had a second version that used DirectX 3 instead of the Intel-developed DINO libraries, which improved the PC version's performance considerably for the time, and had the extended versions of the FMV sequences (albeit in grainy quality) compared to the Sega CD version. The second version of the PC port would later serve as the basis of the Gem Collection port of the game, which featured improved high-color versions of the game's FMVs and the frame-rate during Special Stages was bumped up to 30 FPS, but came with some downsides such as missing or broken effects, lower-quality sound effects, the Debug Mode does not work properly.
  • The 2011 Retro Engine remaster features native widescreen resolution, runs at 60 FPS across the board (even during the Special Stages), has the highest quality FMVs over prior releases, added the ability to use Sonic the Hedgehog 2's Spin Dash mechanics while keeping the original as an option, revamped the game's save feature, has both the North American and Japanese soundtracks, and added Tails as a unlockable playable character.
• Violation of Common Sense:
  • The fake Smashing Hallway Traps of Doom of Wacky Workbench. Standing on one will take you somewhere. One is required to go to the robot transporter in Zone 1.
  • During the Metal Sonic race, it's sometimes a good idea to intentionally let him get ahead of you briefly, as when he does his "charge" move, he is both a lot faster than normal and damages you if you touch him. After a couple seconds of this, he will have to "catch his breath" (metaphorically speaking) and will both slow down a lot and become safe for Sonic to just run through him and get ahead.
• Warm-Up Boss: Palmtree Panic's boss, the EGG-HVC-001, is a slow moving mini-mecha that can't even directly attack you; its only defense is two arms with easily dodgeable Bumpers attached, and otherwise its only "attack" is inflicting collision damage, and it loses an arm with two of the three hits it takes to mow it down, which can be easily done in less than 15 seconds—or if you want to be even quicker, you can just move right behind the boss and attack it directly from behind as soon as it lands (and it can't turn around to retaliate). On top of all that, due to an invincibility box at the beginning of the level, you can actually defeat the boss without being in any danger at all of being hurt. It's easier than Egg Mobile-D.
• A Winner Is You:
  • Finish the game with the good ending, or watch the good ending through Visual Mode. A screen will show up telling you that "YOU ARE GREATEST PLAYER", and Sonic, Amy and Metal Sonic will run across the screen from time to time. This screen was removed in re-releases.
  • The Tails ending in the remake is some explosions and then a credits roll. You do not even get a congratulations. At the very least, the credits for Tails are of the staff who worked on the remake, but it's still kind of disappointing.
• Wheel o' Feet: As usual for the classic games, Sonic's feet fit this trope. When performing the Super Peel-Out, however, Sonic goes up to eleven, causing his feet to form a Mobius strip.
• Word Salad Lyrics: In contrast to "Sonic Boom", which is a clear Bragging Theme Tune about how awesome Sonic is, most of the lyrics to "Sonic-You Can Do Anything" and "Cosmic Eternity" make no sense whatsoever, with the latter sounding more like an abstract, high energy workout tune than a motivational hero song.^note "Cosmic Eternity" starts to make more sense once you get the context- it's a motivational song telling the listener to believe in him/herself, no matter the situation. It ties in to the courage word salad found on the box art. "Sonic- You Can Do Anything" is, however, 75% word salad lyrics, but the chorus again ties into the theme of courage/believing in oneself. The entire thing, in Kefka's words, sounds more like a self-help book.
• You Are Already Dead: During the Metal Sonic race, if you get hit by the spikes (which Metal Sonic can just plow through) and knocked back, or, worse, try to climb up a slope but fail and roll backwards, you can usually kiss your ass goodbye as Eggman is trailing right behind you with an instant death laser. Also, as soon as one of the racers crosses the "finish line", a gate slams shut and leaves the loser trapped outside to die from the laser, meaning if Metal Sonic beats you by even a foot, you're screwed.
• Zonk: Though Bad Futures being interesting levels is a consolation, Future signposts largely exist to distract and roadblock the player from their true objective of fixing the Past. Especially later, accidentally going to a Bad Future at any point in a level can result in being unable to get back to a point where the Past objectives (or even Past Signposts at all) can be reached. As the game goes on, they start being placed in more and more devilish locations, such as just before or after a segment in which is it difficult to stop running, or right after spinning a Past signpost (which are used up after being hit), punishing careless or unobservant players. However, you can simply ignore all of this if you are able to get all the Time Stones though, which automatically triggers the Good Future for the rest of the game.