Rachel leads Percy and Annabeth through the Marriott entrance into the Labyrinth. Annabeth opens the sealed door with a half-blood touch, and the group passes a chained Cyclops skeleton that unsettles Percy because Tyson and Grover are still missing. At a crossroads, Rachel sees dangers Percy and Annabeth cannot see and chooses the apparently safest path, proving that her unusual sight can guide them through the maze.

The passage soon becomes a trap. Kelli, two snake-women warriors, and a Laistrygonian giant capture the group, using Rachel as a hostage and forcing Percy toward an underground arena. The captors hint that their host is Percy’s “brother,” then separate Percy from Annabeth and Rachel so he can be used as entertainment.

Inside the skull-filled arena, Percy finds Luke presiding beside Antaeus, a huge son of Poseidon and Gaea who rules the place and demands violent games before allowing Luke’s army through his territory. Percy watches a centaur executed, then is identified by Luke as another son of Poseidon. Antaeus intends Percy’s death as tribute to Poseidon, but Percy is horrified by the idea that his father would be honored with slaughter.

Percy first defeats a snake-woman gladiator too quickly for Antaeus’s taste, then faces Ethan Nakamura, a one-eyed demigod trying to prove himself worthy of joining Luke. Percy outlasts Ethan and disarms him, but refuses to kill him despite Antaeus’s order. To save Ethan and his friends, Percy challenges Antaeus directly and makes him swear on the River Styx that Percy’s victory will free them all.

During the duel, Percy discovers that Antaeus heals whenever his wounds touch the earth because Gaea, his mother, sustains him. Percy uses the hanging chains and skulls above the arena to lure Antaeus off the ground, tangles him in the chains, and kills him while he is suspended beyond the earth’s reach. Luke refuses to honor Antaeus’s oath after Antaeus dies and orders the monsters to kill Percy, though Luke spares Annabeth for a private conversation before his planned triumph.

With the arena turning against them, Percy finally uses Quintus’s mysterious dog whistle. It shatters into ice and summons Mrs. O’Leary, who attacks the guards and throws Kelli into Luke’s lap. Rachel identifies the far exit as the correct route, and Percy, Annabeth, Rachel, Ethan, and Mrs. O’Leary flee while Luke’s disorganized army scrambles after them.