The game begins with Sonic and friends celebrating Sonic's 20th birthday, when a mysterious creature known as the "Time Eater" appears and uses "time holes" to scatter Sonic and his friends through different points in Sonic's history. Sonic must find his friends, by teaming up with his past self, and put a stop to Eggman's all new sinister plot.
In Sonic Generations you play as both the Modern Sonic and the Classic Sonic, each having their own stages, and each play differently. Modern Sonic has 3D stages, and the gameplay is more fast paced. Classic Sonic has 2D stages, and is more of a platformer, as in past Sonic the Hedgehog games.
The Nintendo 3DS version of Sonic Generations features 6 all new stages unique to the 3DS edition, but lacks 2 additional levels that other versions of the game have. The only stage that appears in both the 3DS version and the versions developed by Sonic Team is Green Hill Zone.