Percy wakes in a salvaged rowboat with Annabeth after the destruction of Clarisse’s ship. Annabeth confirms that Tyson is probably dead, and Percy is crushed by guilt over ever being ashamed of him. As they drift through the Sea of Monsters, Annabeth estimates they have less than a day to save Grover. Percy presses Annabeth about Chiron’s secret, and Annabeth reveals the prophecy about the next half-blood child of the Big Three to reach sixteen: that person will decide whether Olympus is saved or destroyed. Percy realizes Kronos may want to recruit him rather than kill him.

A seagull leads them to a luxurious island resort called C.C.’s Spa and Resort. Though Percy and Annabeth are wary, exhaustion and hunger make them follow the staff inside. The place feels enchanted and persuasive, especially its owner, C.C., who quickly separates Annabeth from Percy. C.C. plays on Percy’s insecurities, shows him an idealized version of himself, and offers a magical makeover drink.

Percy drinks the mixture and is painfully transformed into a guinea pig. C.C. reveals that she is Circe and that she turns men into animals because she despises them. She throws Percy into a cage with other long-imprisoned guinea pigs and plans to ship him away as a classroom pet. When Annabeth returns, Circe tries to seduce Annabeth with knowledge, power, immortality, and a life of sorcery, appealing to Annabeth’s ambition and frustration at how women are overlooked.

Annabeth only pretends to be tempted long enough to act. She finds Hermes’s multivitamins in Percy’s clothes, swallows one, and becomes immune to Circe’s magic. When Circe and her attendants try to attack, Annabeth threatens Circe with her knife and dumps the vitamins into the guinea pig cage. Percy and the other transformed men turn human again. One of them is the pirate Blackbeard, Edward Teach, a son of Ares, and he leads the newly freed men in chasing Circe and trashing the resort.

While the island falls into chaos, Percy and Annabeth flee to the docks. Needing a fast escape, Percy chooses Blackbeard’s old three-masted ship, the Queen Anne’s Revenge. Though there is no crew, Percy instinctively commands the sea and rigging, and the ship obeys him. He and Annabeth sail away deeper into the Sea of Monsters with a new vessel, a clearer sense of Percy’s dangerous destiny, and their quest still racing against time.