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. the most noticeable addition to this game is appearance of Miles "Tails" Prower, Sonic's sidekick, and a two player competitive mode.
Apparently, time restrictions necessitated dropping some features from the final game, such as the time travel aspect. 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.
Levels can be played using additional characters when attached to a passthru cartidge that was released by Sega later, called "Sonic and Knuckles". 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.