Sonic Generations, developed by SEGA, is a crossover game of modern and classic Sonic. The game starts with a new enemy, Time Eater, who kidnaps Sonic’s friends and traps them in different eras of the past. Sonic has to travel back in time to meet his 1990s incarnation to defeat Time Eater and restore the space-time continuum.

The game includes levels from Sonic’s old games, split into "Classic" 2D levels and "Modern" 3D counterparts. The Sonics from different eras have unique abilities, and the Skill Shop allows players to obtain new skills. The game features two boss fights and a Doppelganger Race in each zone. When not playing levels, the Sonics reside in the White Zone, an open world that serves as a hub to access the regular zones.

Sonic Generations offers a lot of unlockable content, such as music, art, special skills, and even the original 1991 Sonic the Hedgehog game in some versions. Overall, Sonic Generations is an excellent game for both new and old fans of Sonic, with its engaging storyline, unique gameplay, and nostalgic feel.