Percy, Annabeth, and Grover move through the Underworld’s Fields of Asphodel, where Percy sees the lines leading to punishment and to Elysium and realizes how few souls reach happiness. As they head toward Hades’s palace, Grover’s winged shoes suddenly drag him away into a side tunnel. Percy and Annabeth chase him and discover a vast chasm Percy recognizes from his dreams: the entrance to Tartarus. Grover narrowly survives when the shoes fly off into the pit, and the three hear an ancient, evil voice rising from below before they flee back to safer ground.

They continue to Hades’s fortress, passing its ominous gates, grotesque garden, and Persephone’s tempting pomegranate trees. Inside, Percy notices his backpack has become unnaturally heavy, but he has no time to investigate. In the throne room, Hades confronts Percy and accuses him of stealing both Zeus’s master bolt and Hades’s missing helm of darkness as part of Poseidon’s plot to start a war. Hades insists he does not want war, explaining that the Underworld is already overcrowded, and says he allowed Percy to reach him because he expected to force the stolen items back from him.

Percy denies everything, but when Hades orders him to open his pack, Percy finds Zeus’s master bolt inside. That discovery proves Percy has been manipulated by someone else and makes him realize Hades is not the thief he expected. Hades then reveals Sally Jackson, still alive but imprisoned in magical stasis, and offers to release her only if Percy returns the helm. Percy understands that he cannot hand over the bolt, because stopping the coming war matters more than making a bargain that would strengthen Hades.

Faced with only three pearls for four people, Grover and Annabeth each volunteer to stay behind so Percy can rescue his mother. Percy refuses to abandon either friend or to doom Olympus by surrendering the bolt. Accepting the prophecy that he will fail to save what matters most, Percy gives pearls to Annabeth and Grover, promises Sally he will return for her, and defies Hades. When the skeleton army and Furies attack, the three smash the pearls and are enclosed in protective bubbles that rise out of the Underworld.

The pearls carry Percy, Annabeth, and Grover up through rock and water until they burst onto the surface in Santa Monica Bay on the morning of the summer solstice. Percy sees Los Angeles burning from the earthquake-like rage Hades unleashed after their escape. Safe for the moment but now certain he has been set up, Percy knows he must reach Olympus quickly, return the master bolt, and confront the god who deceived him.