In a past version of Green Hill, Classic Sonic is running until he hears a noise. Suddenly, the mysterious entity known as the Time Eater appears in the sky and the screen washes white.

In the present day, Sonic celebrates his birthday with his friends until the Time Eater disrupts the party, sucking everyone through various "time holes" and scattering them across different points in Sonic's past. Sonic tries to stop the monster, but is easily overwhelmed by its power. After regaining consciousness, Sonic finds himself in a strange dimension known as White Space, a realm where time and space end up after they have been 'erased' by being drained of color and life. Sonic rescues his best friend Miles "Tails" Prower and as they search for their friends, they encounter versions of themselves from the past, referred to as their "Classic" selves. As the two Tails determine that Time Eater's actions are damaging time and space itself, which is why it is white and lifeless, both "Classic" and "Modern" Sonic race through their history, restoring time to normal and rescuing their friends. They also realize at one point that the Time Eater's weakness are the Chaos Emeralds.