Sonic Generations

Sonic Generations is a crossover of modern Sonic with his 1990s incarnation. The blue hedgehog is attacked by a new enemy called Time Eater, who tries to change the past to help Dr. Robotnik. Time Eater kidnaps Sonic’s friends and imprisons them in the various eras of the past. Sonic has to travel back in time to meet the original Sonic and Tails from the 16 bit game series. Old and new versions of characters team up to defeat Time Eater and Robotnik and restore the space-time continuum.
Sonic will have to revisit the levels from his old games. Each zone is split into “Classic” 2D levels from Sega Genesis and Dreamcast eras, and their “Modern” 3D counterparts from the 2000s. Sonics from the different eras also have their unique abilities, and there’s the Skill Shop, where the characters can obtain new skills. Each zone includes two boss fights and a Doppelganger Race, which is a time trial. When they're not on the levels, the Sonics reside in the White Zone, an open world that serves as a hub to access the regular zones.
Sonic Generations includes a lot of unlockable content, such as music, art, special skills and, in some versions, even the original 1991 Sonic the Hedgehog game.