Adventure through the white void

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

After restoring the worlds and collecting all the Chaos Emeralds, they discover that the mastermind behind the Time Eater is Eggman and his classic self, Classic Eggman, who was faking himself to be a victim of the Time Eater all along. He reveals that while drifting through space following the events of Sonic Colors, Eggman discovered the Time Eater, thinking about the idea to use its powers and decided to join forces with his past self to harness its power. Although the Time Eater manages to nearly defeat the two Sonics, the support of their friends and the power of the Chaos Emeralds allow them to transform into Super Sonic and Classic Super Sonic, and confront both versions of Eggman. Despite attacking with missiles, warping arms and slowing down time, the Sonics penetrate the Time Eater's core, destroying it. The heroes return to the present and continue celebrating Sonic's birthday. After the party, Classic Sonic and Classic Tails]travel back to their own world as everyone bids their farewells.

A post-credits scene shows the two Eggmen lost in White Space. The two argue as they search for an exit, discuss whether or not they ever defeat Sonic and contemplate if they should go back and get their teaching degree as opposed to continuing their misdeeds.