The story begins with Classic Sonic in Green Hill. After completing Act 1, a purple vortex in the sky appears and surprises Classic Sonic. After that, it goes all the way to the present day and Sonic is invited to a birthday party by Tails. However, Sonic arrives early, so Tails tells him to wait. But then a vortex sucks Tails into it. Sonic jumps in but is "attacked," though he manages to get in anyway. Sonic sees a white world and wonders what's going on. Modern Sonic rescues Tails after restoring Green Hill, and Tails tells him that he sees a big city in nighttime and a giant mushroom, so Sonic decides to go check those places out, being followed by Classic Sonic, who is believed by Tails to be Modern Sonic.