Hades is about to start his war. Percy is standing on the shore of the Pacific Ocean and sees Ares in the distance. It seems like the god of war has orchestrated this upheaval. Percy confronts the god, who reveals that he has Hades’s helm of darkness. Percy challenges Ares to a duel, and the helm and the master bolt will go to the winner. It is a difficult fight, and it attracts the attention of the mortal police. Using the power of the ocean, Percy stabs Ares’s ankle, and this wins him both the fight and Hades’s helm. He returns the helm to the Furies, who take it to Hades. The police recognize Percy as the fugitive from New York. Percy says he was abducted and swears that the only thing he wants is to return to New York to be with his step-father.
When Percy arrives in New York, he returns the master bolt to Zeus, who rewards Percy by deciding not to kill him. When Percy suggests that Kronos has orchestrated this entire dispute, Zeus refuses to listen and declares the matter closed. He then leaves in a flash of lightning, leaving Percy alone with his father, Poseidon, in Olympus. Poseidon explains that he is proud of Percy and that Hades has released his mother in return for his helm of darkness. It is clear that he will never join Percy and his mother in New York. However, he has left something for Percy in New York.
Back in his mother’s home, Percy realizes that Gabe has hit his mother at some point. When he goes to his room, he finds the head of Medusa, which was returned to sender. He offers to use the head to free his mother from Gabe, but he realizes that his mother is independent. She can save herself. Percy returns to Camp Half-Blood and leaves the head with his mother. She turns Gabe into a statue and uses the proceeds of its sale to enroll in university. She also obtains a spot for Percy at a private school that fall.
Back at Camp Half-Blood, Percy is forced to choose what he will do in the fall: enroll at Camp Half-Blood year round or go to private school. He is unable to come to a decision. Percy goes to practice with his sword and sees Luke, who is using a sword with one celestial bronze edge and one steel edge. Its name is Backbiter and it can be used against gods, monsters, and mortals, which seems wrong to Percy. The two go into the woods and Luke attempts to kill Percy with a scorpion. Before he does so, he explains how he stole the master bolt for Kronos, lost it to Ares, and later summoned monsters to kill Percy. The shoes he gave Percy to aid him on his quest were in fact designed to deliver Percy to Kronos.
When Luke leaves, the scorpion attacks Percy. However, he survives the sting and wakes up in the hospital. Chiron explains that Percy may become the greatest of all the heroes. However, first he must train. Grover has been granted his quest. Annabeth visits and explains that she has decided to return to life with her father. Percy decides that he, too, will return to the mortal world.