While guarding the Master Emerald, Knuckles encounters an unknown enemy, but before he realizes it, this new foe steals the Master Emerald, causing Angel Island to fall from sky. This new creature’s name is Nazo, a sentient being created from the negative energy of the Chaos Emeralds. His plan is to steal the Emeralds to become more powerful, and so it's time for Sonic and his friends to stop him before it's too late.