Sonic the Hedgehog 2

Sonic the Hedgehog 2 is the second game in the Sonic the Hedgehog series released for the Sega Genesis in 1992. The storyline is as follows: Dr. Robotnik has captured all of the animals of the world and it is up to Sonic to free them. Initial information from Sega stated that time travel would be involved. This was not a part of the game when it was released, and this could have been a mix-up with Sonic CD[?] or the concept was simply passed on into it. The most noticeable addition to this game is appearance of Tails, Sonic's sidekick, and a two player competitive mode. It was also possible for Sonic to become Super Sonic[?], turning his fur yellow and spikier, an obvious homage to the Dragon Ball Z "Super Saiyan" effect.

Apparently, time restrictions necessitated dropping some features from the final game. Study by fans of the internals of the game using emulators and a debug mode revealed hidden incomplete levels and unused sounds and graphics. Some of these resurfaced in Sonic the Hedgehog 3.

"Knuckles the Echidna in Sonic the Hedgehog 2" is a game created by locking "Sonic the Hedgehog 2" to a passthru cartridge that was released by Sega later, called Sonic & Knuckles[?] (this being the only passthru cartridge ever). The resulting game is identical to "Sonic the Hedgehog 2", with the exception that you play as Knuckles the Echidna throughout it. Some fans believe that Sonic 2 was created with foreknowledge that such an add-on device would be added later and that the unfinished levels were somehow connected with this fact, but this seems unlikely. While there are some similarities in the hidden level data to other Sonic games, a more likely explanation is that unfinished work from Sonic 2 was used in an inevitable sequel game.