Percy is sealed in Hades's airless dungeon, unable to escape even after Riptide returns to his pocket. While trapped, Percy dreams first of Rachel at her family's beach house, then of St. Louis, where Typhon has reached the Mississippi and is battling Zeus, Artemis, and possibly Apollo while destroying the city.

Percy wakes when Nico enters the cell to rescue him. Percy distrusts Nico after the betrayal, but Nico swears he did not know Hades's plan and puts the skeletal guards to sleep so they can flee. Nico exhausts himself using his powers, and Percy drags him through the Underworld until alarms sound.

Mrs. O'Leary finds them in the Fields of Asphodel and carries Percy and Nico past the gates and toward the River Styx. At the river, Nico explains that Percy must prepare or the water will destroy him. The ghost of Achilles appears, reveals that Luke also came to the Styx, and warns Percy that the river grants power but creates a mortal weak point tied to the hero's flaws.

Despite Achilles's warning, Percy chooses the small of his back as his vulnerable point and enters the Styx. The river nearly dissolves Percy, but memories and voices of loved ones help him hold on; Annabeth's voice and image become the lifeline that anchors Percy to life. Percy survives, emerges burned but quickly heals, and realizes the ritual worked.

Hades arrives with an army of dead Roman soldiers and British redcoats and orders Percy destroyed. Percy unleashes the River Styx, then charges through the army with overwhelming speed and strength, untouched by bullets or blades. Percy defeats the army, pins Hades, demands information about the trap, but Hades vanishes before answering.

Afterward, Percy frees the tormented souls in Hades's robe by dropping it into the Styx. Percy tells Nico to return to Hades, learn what the trap is, and try to convince his father to help Olympus. Nico wants to fight beside Percy, but Percy sends him back, then leaves with Mrs. O'Leary to find Luke and begin the war.