At breakfast, the campers learn that an Aethiopian drakon tested Camp Half-Blood’s magical borders overnight and withdrew only after Lee Fletcher and Apollo campers drove arrows into it. Quintus warns that such attacks will become more frequent, intensifying fears that Luke’s monster army is preparing to invade while camp numbers are shrinking.

Chiron brings Grover to Percy, and Annabeth breaks dining rules to join them at Poseidon’s table. Annabeth explains that she and Clarisse have been investigating the Labyrinth, an enormous, living maze beneath the mortal world that can connect distant places. She reveals that Chris Rodriguez, a former camper who joined Luke, emerged from the Labyrinth insane and muttering about string, suggesting Luke is searching for Ariadne’s string to navigate the maze.

Annabeth argues that the Labyrinth may also be Grover’s last chance to find Pan, because it can lead almost anywhere. Grover is terrified of going underground, especially after his past captivity in a Cyclops’s cave, and worries he will lose his searcher’s licence. Later, Percy visits Blackjack, who warns him against entering any maze.

That night, Quintus introduces a new war game: pairs of campers must hunt monsters carrying silk packages, one of which contains gold laurels. Percy is paired with Annabeth, while Grover is paired with Tyson, alarming both of them. In the woods, Percy and Annabeth follow many-legged tracks and pause at the place where they once gave up searching for Nico. Percy tells Annabeth about the Iris-message showing Nico summoning the dead and being urged toward revenge, and Annabeth concludes they must understand the Labyrinth.

Near Zeus’s Fist, Percy and Annabeth encounter Juniper, who is worried about Grover and insists he cannot go underground. Before Juniper can explain more, three giant scorpions attack. Forced against the rocks, Percy and Annabeth squeeze into a crack, but the ground opens beneath them and drops them into a dark stone corridor.

Annabeth recognizes the corridor as part of the Labyrinth and finds the mark of Daedalus, opening a hidden exit. Although Percy and Annabeth think only minutes have passed, the other campers have been searching for nearly an hour. Clarisse wins the game but focuses on what they found, and Annabeth confirms that the passage is an entrance to the Labyrinth—an invasion route directly into the heart of Camp Half-Blood.