Sonic Rebirth is a reimagining of Lone Planet's classic-styled Sonic the Hedgehog video games. It is meant to be a sort of "prologue" to the Vicinity Saga, and thus explains the origins of the various realms it depicts.
One year after the events of Sonic & Knuckles, things seem to be going well for Sonic and his friends... until they find out about Dr. Robotnik's latest plan.
Robotnik has been investigating a mysterious entity that, at its highest level of strength, can destroy entire worlds and remodel them in its own image. His goal is to harness its power in order to remake the universe as he sees fit -- and of course, that means fully mechanized. So he launches his latest project, dubbed "Rebirth", and sends an army of old and new Badniks all around various Mobian islands.
After Robotnik captures some of their friends, Sonic, Tails, Knuckles, and Amy decide to team up with two new heroes -- Chill the Cyberwolf, a tech-savvy expert on alternate realities; and Grudge the Komodo Dragon, a middle-aged philosopher who has studied mystical entities for decades.