The game's plot involves Sonic's friends throwing Sonic a party, but later a big monster named the Time Eater comes to his party and sucks his friends back in time. Then Sonic wakes up in a white limbo, saves Tails, then they meet classic versions of themselves. Then later they fight Metal Sonic, Dr. Eggman, etc. Then at the final battle, they meet a classic version of Eggman. They are controlling the Time Eater to undo their previous defeats, then both Sonics go super, then defeat the Time Eater. After so, the party continues, and Classic Sonic returns home.
Classic Sonic is strictly 2D, like the Genesis days. He has all his moves from Sonic 2, including the Spin Jump, and Spin Dash.
Modern Sonic is in 3D and 2D. His gameplay is like Sonic Unleashed, and Sonic Colors/Colours. Including moves such as the Sonic Boost, and Sonic Drift.
Green Hill (Sonic the Hedgehog)
Chemical Plant (Sonic the Hedgehog 2)
Sky Sanctuary (Sonic the Hedgehog 3 & Knuckles)
Speed Highway (Sonic Adventure)
City Escape (Sonic Adventure 2)
Seaside Hill (Sonic Heroes)
Crisis City (Sonic the Hedgehog (2006))
Rooftop Run (Sonic Unleashed)
Planet Wisp (Sonic Colors/Colours)
Metal Sonic (Sonic CD) Arena: Stardust Speedway
Death Egg Robot (Sonic the Hedgehog 2) Arena: Death Egg
Shadow the Hedgehog (Sonic Adventure 2) Arena: Final Rush
Perfect Chaos (Sonic Adventure) Arena: Station Square
Silver the Hedgehog (Sonic the Hedgehog (2006)) Arena: Crisis City
Egg Dragoon (Sonic Unleashed) Arena: Eggmanland
Time Eater (Sonic Generations) Arena: Center of Time
The game got positive reviews, even more so then Sonic Colors.