The book begins with Percy going to a freshmen orientation class at Goode High School, a school where Paul Blofis, his mom's boyfriend, teaches. While there, he meets Rachel Dare, a clear-sighted mortal whom he met previously at the Hoover Dam in The Titan's Curse. During the orientation, Rachel suddenly runs away to the gym. Percy follows her, and she tells him that something is wrong with the two presenting cheerleaders, Tammi and Kelli.

The people in question enter the gym where they reveal themselves to be empousai. Percy slays Tammi, but Kelli disappears in a burst of fire. Unfortunately, Paul Blofis and some other mortals see the incident forcing Percy and Rachel to escape through a broken window. They both run outside to where Annabeth is waiting. Rachel leaves Percy, telling him to explain everything when he has time. Then, Percy, along with Annabeth make their way to camp.

Arriving at Camp
After reaching camp, Annabeth decides to go talk to Clarisse about something secret, leaving Percy to wander the camp by himself. He stops by the arena and sees a massive hellhound, Mrs. O'Leary. He tries to kill her, but is soon stopped by Quintus, the new sword-master, who reveals that Mrs. O'Leary is his pet. Chiron interrupts their conversation and takes Percy. He leads them to a glade where the Council of Cloven Elders is being held and drops him next to Annabeth, Clarisse and a crying nymph, Juniper, who turns out to be Grover's girlfriend.

Mrs. O'Leary
Mrs. O'Leary.
The council decides that Grover has only one week to find Pan, the lost God of the Wild. Otherwise, his searcher's license will be forfeited. The meeting concludes, and Percy rushes over to clean up his cabin for the cabin inspection. As soon as he reached the Poseidon cabin, he sees his half-brother, Tyson, cleaning up. Tyson hugs him and shows him the repaired shield that latter broke last winter during the fight with the manticore.

Later that night, Percy has trouble sleeping. He realizes that the room is glowing with a faint blue light coming from the saltwater fountain his father had given him last winter. Surrounding it is a rainbow colored light, clearly an Iris-message. He deposits one drachma into the mist, and it shimmers until the figure of Nico di Angelo becomes clear. Nico is conversing with a ghost, asking if there is a way to bring Bianca back to life. The ghost answers that it is possible, but only if an exchange happens — a soul for a soul. The vision vanishes before Percy has time to put in another drachma. In his mind, he has a chilling thought that Nico would come looking for him.

The next morning, in a camp game designed by Quintus, Annabeth and Percy are attacked by three giant scorpions. They try to find a place to hide, and squeeze between two boulders. They note that it must be the entrance to a tunnel. When they emerge from the entrance, it is revealed that they have actually discovered an entrance to the Labyrinth, a magical underground maze, under Zeus's Fist within the heart of the camp. Annabeth explains that this is the center of the secret mission Clarisse had undergone during The Titan's Curse — finding an entrance to the Labyrinth. She also explains that Luke might be planning to use it as an invasion route.

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Percy Jackson, one of the members of the quest.
Into the Labyrinth
Annabeth leads a quest with Percy, Grover and Tyson to find Daedalus, the maze's creator. As soon as the group step into the maze, it confuses them, shifting its walls, narrowing its passageways, and closing off exits and entrances. They make it into a Roman-styled room where they find the minor god Janus. He offers Annabeth a key to one of the doors behind him. He forces her to make her choice when they are saved by the Queen of Olympus Hera. She tells them that she has only delayed Annabeth's choice, and adds that the real reason for her visit was to offer them a wish. Annabeth wishes for a way to navigate the