Summary
The Chao have vanished, prompting Sonic the Hedgehog and Miles "Tails" Prower to spring into action in search of them. It soon becomes clear that Eggman Nega—once again pretending to be Doctor Eggman—has taken the Chao and hidden them in a spooky mansion. His goal is to feed them to a powerful inter-dimensional creature known as the "Ifrit," which requires the Chao to gain invincibility. Once that happens, he intends to unleash the Ifrit upon the world to bring about its destruction. To open the portal to the Ifrit's realm, however, the seven Chaos Emeralds are needed. Eggman Nega secretly hires Rouge the Bat to gather them. The Master Emerald has also disappeared (again), and Knuckles the Echidna joins forces with Rouge to track it down.
At the same time, Silver the Hedgehog returns from a future now in ruins, caused by the Ifrit. Determined to create a better future, he goes on a mission to find and hide any remaining Chao. Espio initially suspects Silver of having evil intentions, but soon discovers that Silver is actually trying to save the world. With this revelation, Espio and Silver team up to stop the impending disaster. The real Dr. Eggman sends Metal Sonic to locate Shadow and informs him of Nega’s plan. Nega learned about the Ifrit from Professor Gerald Robotnik’s journals, and Shadow and Metal Sonic set out to retrieve the Chaos Emeralds before Nega can. Eggman uses Metal Sonic as a communication device to assist Shadow along the way.
All the teams converge at the Mystic Haunt Zone, where the portal to the Ifrit’s world is located. Although Rouge only managed to collect six of the Emeralds, the portal still opens, and Nega unleashes a new version of Metal Sonic—Metal Sonic 3.0—to awaken the Ifrit. Even as the Ifrit takes control of the minds of Sonic, Tails, Knuckles, and Rouge (depending on the player’s character), the other teams manage to defeat it. Shadow and Metal Sonic ultimately destroy the Ifrit and close the portal, but both they and Eggman Nega are trapped in the Ifrit’s dimension. In a final act of sacrifice, Metal Sonic rips open his body to reveal the seventh Chaos Emerald. Shadow uses it to teleport back with Metal Sonic, leaving Nega stuck beneath the rubble.
Sonic and Tails rescue all the Chao and take them to a Chao Garden, where they relax and recover. Knuckles locates the Master Emerald using the Emerald Detector he and Rouge had stolen from Eggman Nega. Rouge, ever the opportunist, steals the Master Emerald and makes a quick escape. Silver returns to what he hopes is a brighter future. Meanwhile, Espio must face Vector the Crocodile, who spent their client’s advance payment on overdue rental fees.