Percy, Annabeth, and Tyson arrive at Camp Half-Blood and immediately find Clarisse and other campers fighting two huge bronze, fire-breathing bulls on Half-Blood Hill. Percy quickly realizes the monsters are moving around Thalia's tree and crossing areas the camp's magical borders should protect, which means something is wrong with the defenses. Annabeth identifies the attackers as the Colchis bulls and warns Percy they are too dangerous to face normally, but Percy charges in anyway when he sees Clarisse and her patrol losing ground.

Clarisse tries to organize a phalanx, and Annabeth distracts one bull by turning invisible, but the defense collapses when both bulls press the attack. Percy pulls Clarisse out of danger and wounds one of the monsters, yet he cannot seriously stop them. As the bulls push farther inside the hill toward the camp, Percy understands that the cabins, training grounds, and Big House are all at risk if the monsters break through.

Percy attempts to fight one bull alone, but its fire blast forces him back and leaves him with a badly injured ankle. When a second charge would have killed Percy, Annabeth gives Tyson permission to cross the boundary line into camp. Tyson runs through the bull's fire completely unharmed, then crushes its bronze face with his bare hands and knocks it down, while Clarisse finishes disabling the other bull.

After Annabeth gives Percy nectar, Clarisse angrily insists she had the situation under control, then returns to help the wounded. Percy is stunned that Tyson survived the fire, and Annabeth tells Percy to look past the Mist. Percy finally sees that Tyson has a single eye and learns Tyson is a young Cyclops, one of the unwanted children of nature spirits and a god, and that Cyclopes are immune to fire because they work divine forges.

As the aftermath unfolds, Clarisse tells Percy that Chiron is no longer activities director, Argus has been fired, and a new director named Tantalus is in charge. She then points Percy to the real cause of the disaster: Thalia's pine tree is sick, its needles yellowing and green sap leaking from a puncture wound. Percy realizes someone has poisoned the tree, and that the camp's magical borders are failing because Thalia's protective power is dying.