History VideoGame / SonicTheHedgehogCD

** Knuckles and Amy are playable in the ''Origins Plus'' remake.

* PromotedToPlayable: 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.

-->''[...]'Cause if you try, you can do anything!''
-->'''''YOU CAN'T DO ANYTHING, SO DON'T EVEN TRY.'''''

** 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.[[/note]] some artwork, and a dummied out Special Stage are both hidden in the game.

* PromotedToPlayable: 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.

* UnlockableContent: The Special Stages in Time Attack, the D.A. Garden (the incomplete Sound Test, plus an image of the Little Planet) and the Visual Mode (the Video Gallery).

* StealthPun: The reason levels in this game are called Zones instead of Acts? That's because this game is (about) time travel.[[note]]That is, "Palmtree Panic ''Time Zone'' 1", for example.[[/note]]

* TakeThat: [[AllThereInTheManual The name of the first boss]] is the EGG-HVC-001. HVC-001 was the model number of Creator/{{Nintendo}}'s own [[UsefulNotes/NintendoEntertainmentSystem Famicom]] console. As a bonus stealth insult, the HVC-001 [[WarmUpBoss 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.

* SuspiciouslySimilarSubstitute: According to the developers, several of the areas in this game were designed as more surreal versions of the zones from ''VideoGame/SonicTheHedgehog1'', and even appear in the same order: PalmtreePanic is GreenHillZone, Collision Chaos is Spring Yard Zone, Tidal Tempest is Labyrinth Zone, Stardust Speedway is Starlight Zone, and Metallic Madness is Scrap Brain Zone. The similarities are less apparently because Quartz Quadrant and Wacky Workbench aren't based on anything, while there's no equivalent to Marble Zone (it was supposed to be Dubious Depths, the scrapped second round).

* SuspiciouslySimilarSubstitute: Except for Wacky Workbench (and possibly Quartz Quadrant), the various areas of the game all look like enhanced versions of the zones from [[VideoGame/SonicTheHedgehog1 the first Sonic game]]: PalmtreePanic is essentially GreenHillZone, Collision Chaos is Spring Yard Zone, etc.

* SuspiciouslySimilarSubstitute: Except for Quartz Quadrant and Wacky Workbench, the various areas of the game all look like enhanced versions of the zones from [[VideoGame/SonicTheHedgehog1 the first Sonic game]]: PalmtreePanic is essentially GreenHillZone, Collision Chaos is Spring Yard Zone, etc.

** Even the 2011 remaster, when compared to the 2013 remasters of ''Sonic 1'' and ''2'', isn't safe from 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[[/note]], there is no option to play as Knuckles or Sonic and Tails together, 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.[[/note]]

In May 2021, Sega announced that ''Sonic CD'' and the other mainline Genesis ''Sonic'' games would be [[CompilationRerelease 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 to release 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.

In May 2021, Sega announced that ''Sonic CD'' and the other mainline Genesis ''Sonic'' games would be [[CompilationRerelease 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. 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.

** The Bad Future themes of the US soundtrack don't sound grimmer than the Good Future ones.

* SuspiciouslySimilarSubstitute: Except for Quartz Quadrant and Wacky Workbench, the various areas of the game all look like enhanced versions of the zones from ''[[VideoGame/SonicTheHedgehog1 the first Sonic game]]'': PalmtreePanic is essentially GreenHillZone, Collision Chaos is Spring Yard Zone, etc.

** The familiar ''Sonic'' drowning music is absent in this game; the regular level music continues playing as the drowning timer ticks down.

North America (The United States and Canada) later got the game with the redone soundtrack on November 19, 1993.

* CuttingOffTheBranches: [[Videogame/SonicGenerations Sonic Generations]] and [[Videogame/SonicTheHedgehog4 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 TimeyWimeyBall, even this may not strictly be true.

* InstantWinCondition: 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 BadEnding. 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''. And if you're skilled, the earliest you can achieve this is the end of ''Quartz Quadrant Zone 1''; which is roughly halfway thru the game. Even better, any remaining levels before the end will show a Good Future for the rest of the game whenever you go there.

* TheGlomp: [[StalkerWithACrush Amy]] will try to do this to Sonic twice: once in Palmtree Panic, and again in Stardust Speedway.

* InstantWinCondition: 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 BadEnding. 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''. And if you're skilled, the earliest you can achieve this is the end of ''Quartz Quadrant Zone 1''; which is roughly halfway thru the game.
** Furthermore, getting every Time Stone also causes the Good Future to be automatically triggered for the rest of the game whenever you go to the Future.

* SpeedRun: This is the first game in the series to feature a Time Attack mode. This is also how you unlock some of the extras.