Sonic Generations

Sonic Generations (ソニック ジェネレーションズ: 白の時空) is a platform game in the  series. It serves as a retrospective of the series.

Story
As Sonic celebrates his birthday with his friends, a mysterious new enemy called the Time Eater appears, sucking everyone through various 'time holes' and scattering them across different places in Sonic's history. As Sonic searches for his friends, he ends up meeting his younger self, Classic Sonic. Both Modern and Classic Sonic decide to collaborate and race through their history in order to save their friends and fix the time stream.

Gameplay
The game features two versions of Sonic: the modern version, and the "classic" version of the character from the original Genesis games. Each level is modeled on a level from a previous Sonic game, and comes in two "Acts". "Act 1" is played as Classic Sonic, and "Act 2" is played as Modern Sonic. When playing as Classic Sonic, the gameplay closely resembles that of the original Genesis Sonic games; when playing as Modern Sonic, the gameplay is similar to more recent Sonic games.