(copied from Wikipedia):
Following the war over Eggman's global conquest,[a] the doctor has lost his memory and ended up in a remote village where he has taken on the more friendly moniker of Mr. Tinker. Sonic learns that Neo Metal Sonic is now leading the Badniks, plotting to take over the world in Eggman's stead. While Sonic and his friends fight Neo Metal Sonic and his army on Angel Island, an Eggman admirer named Dr. Starline kidnaps Mr. Tinker and successfully reverts him back to Eggman after a defeated Metal Sonic returns to him.
While Tangle helps Whisper apprehend her former comrade turned criminal Mimic, Eggman hatches a new scheme involving the Metal Virus, which turns organic life into mindless robots dubbed "Zombots", but it quickly mutates beyond his control. Starline brings in the Deadly Six in hopes that they can control the Zombots, but they betray him. This leads to a falling out between Starline and Eggman and forces the latter to help Sonic and his friends cure the world.
