Sonic chills out in Station Square until a recent riot invites him to face off against a strange enemy made of water. When the creature is defeated, Sonic runs into Tails, who in turn finds a Chaos Emerald on his journey. The duo go to the Mystic Ruins. On the way, they meet Eggman, who introduces the two to 'Chaos', the water monster Sonic fought earlier. By nabbing the Chaos Emeralds, Eggman feeds them to Chaos, which transforms into a bigger creature after each Emerald given. If Chaos absorbs all of the seven emeralds' power, he'll be invincible. What's worse, Eggman plans to use this to destroy Station Square and to build his own city, 'Robotnikland'. As Sonic and Tails venture to stop him, they encounter other characters, who each have a place in the story...