After Sonic the Hedgehog comes to Earth, he hides in the town of Green Hills, and is helped by the town's Sheriff to escape the evil Dr. Robotnik who's hired by the government to capture him.
Upon learning that Dr. Robotnik has teamed up with Knuckles in the hope of finding an emerald with supernatural powers, Sonic sets out with Tails on a dangerous mission to locate the emerald.
It follows a black hedgehog named Shadow, who appears and begins to use cunning methods to seek revenge, relying on his extreme intelligence.