In Mobotropolis, the gods Jules and Aleena have a son named Sonic. While the other gods are joyful, Robotnik plots to overthrow Jules and rule Christmas Island. Turning to the visions for help, Robotnik learns that in eighteen years, a planetary alignment will allow him to free the Dark Gai to conquer Christmas Island, but only if Sonic does not interfere. Robotnik sends his demon minions, Scratch and Grounder to murder Sonic, providing them with a potion that can strip a god of immortality. The two kidnap the baby and take him down to the valley where they feed him the potion, but before Sonic drinks the last drop, Locke the Echidna pass nearby and startle the robots, causing them to drop the bottle and spill the last drop. Sonic is stripped of immortality hedgehog but retains his god-like strength. Scratch and Grounder attempt to murder the baby, but Sonic easily overpowers them. Scratch and Grounder decide not to report their failure to Robotnik.
Years later, the teenage Sonic becomes an outcast for his inability to control his strength and wonders where he came from. After his foster dad reveals the Christmas Island necklace, he found him with, Sonic decides to visit the Temple of Jules for answers. The temple's statue of Jules comes to life and reveals all to Sonic, telling him that he can earn back his godhood by becoming a "true hero." Jules sends Sonic and his forgotten childhood friend Tails to Athair the Echidna who is known for training heroes. Athair has retired in frustration after none of his past students managed to earn a constellation in the sky. Jules obliges Athair to train Sonic anyway. After completing the training, Sonic, Tails, and Athair head towards Mobotropolis. On the way, they meet Sally Acorn, a sarcastic damsel whom Sonic saves from the centaur. After Sonic and the others leave, Sally is revealed to be Robotniks' servant, who was trying to recruit the centaur and had sold her soul to Robotnik previously to save a lover who then left her. When Sally mentions Sonics' encounter, Robotnik realizes that Scratch and Grounder failed him and plots to finish off Sonic properly.
Arriving in Mobotropolis, Sonic is met with skepticism by the locals, but then Sally appears, saying that two boys have become trapped under a large rock in a gorge. Sonic lifts the rock and rescues the boys, unaware that they are Scratch and Grounder in disguise, and unwittingly releases the Hydra. Much to Robotnik's chagrin, Sonic defeats the monster and becomes a celebrated hero. Sonic defeats many other monsters, many of which are sent by Robotnik, and his popularity and fortune grow, but Jules tells Sonic that he is not yet a "true" hero and refuses to explain what that means. Saddened and frustrated, Sonic spends a day out with Sally, who realizes she has fallen in love with him. Robotnik learns of this, and on the eve of his takeover, he holds Sally hostage and offers her in exchange for Sonic surrendering his powers for a day. On the condition that Sally will be unharmed, he accepts and is heartbroken when Robotnik reveals that Sally was working for Robotnik all along.
Robotnik unleashes the Dark Gai, who climb Christmas Island and defeat the gods, while the Cyclops go to Mobotropolis to kill Sonic. Sonic uses his wits to defeat the Cyclops, but Sally is mortally injured when a pillar collapse on her. This breaks Robotnik's promise that Sally would not be harmed, so Sonic regains his strength. Sonic and Tails fly to Angel Island, where they free the gods and vanquish the Dark Gai, but Sally dies before he can return to her.
Sonic goes to the underworld and leaps into the River Styx to recover Sally's soul. This act would be fatal for a mortal, but his willingness to sacrifice himself is a sufficiently heroic act to restore his godhood. To Robotnik's shock, Sonic climbs out of the Styx with Sally's soul and knocks Robotnik into it. After reviving Sally, she and Sonic are summoned to Christmas Island, where Jules and Aleena welcome their son home, saying that he has proved himself a "true hero" through the "strength of his heart." However, rather than join the gods on Christmas Island, Sonic chooses to remain on Mobotropolis with Sally. Sally and his friends return to Mobotropolis, where they watch Jules form a constellation in Sonic's honor.
