The morning after Poseidon claims him, Percy is moved into cabin three by himself. Instead of feeling honored, Percy feels isolated and miserable, because the other campers now avoid him and the hellhound attack has made him seem dangerous. Luke begins training Percy even harder, Annabeth grows tense and distracted whenever quests are mentioned, and Clarisse keeps glaring at Percy over her broken spear. Percy’s loneliness deepens when someone leaves a mortal newspaper in his cabin showing that he and Sally are missing after the car wreck, with Gabe implying that Percy may have hurt his mother.

That night, Percy dreams of two godlike figures in blue and green fighting on a stormy beach while a dark voice from beneath the earth calls to him. He wakes to find a real storm approaching Camp Half-Blood, which is unnatural for the protected valley. Grover brings Percy to the Big House, where Dionysus insults and threatens him, then leaves for an emergency meeting on Olympus. Once Dionysus is gone, Chiron tells Percy that the hellhound attack means greater dangers are coming and asks whether Percy will accept a quest.

Percy guesses that Zeus and Poseidon are fighting over something stolen, and Chiron confirms that the missing object is Zeus’s master bolt. Chiron explains that Zeus believes Poseidon arranged for a mortal hero to steal the bolt, and now that Poseidon has publicly claimed Percy, Percy has become the obvious suspect. With the summer solstice only ten days away, Zeus demands the bolt’s return and Poseidon demands an apology. Chiron warns that if the bolt is not recovered, war among the gods will devastate the world and Percy will be the first target of Zeus’s anger.

Before Chiron will say more, Percy must consult the Oracle. In the attic, Percy finds a decaying female body that serves as the Oracle’s vessel. Through visions of Gabe and his poker friends, the Oracle prophesies that Percy will go west, face a god who has turned, recover the stolen item, be betrayed by a friend, and fail to save what matters most. Percy is shaken by the final lines and hides them when he reports back, telling Chiron only that he is supposed to go west and return what was stolen.

Chiron then explains his theory that Hades is behind the theft. He reasons that Hades likely used a servant to steal the master bolt, hid it in the Underworld, sent the Fury and the hellhound after Percy, and may even have a spy inside camp. Percy realizes that going to the Underworld may also be his only chance to learn what happened to Sally. Chiron says the gods cannot directly enter one another’s realms to solve the crisis, so a hero must act instead. Percy accepts the quest, Grover overcomes his fear and agrees to go with him, and Annabeth reveals that she has already volunteered as the third companion. Chiron tells them to pack for an overland journey to Los Angeles, because Percy cannot risk flying through Zeus’s domain.