Percy, Annabeth, and Grover return to Camp Half-Blood as rare heroes who completed a quest alive, and the camp honors them with a feast, a procession, and the burning of their burial shrouds. Grover receives his searcher’s license for his bravery. Percy settles back into Cabin Three, reflects that Poseidon seems to be watching him with pride, and receives a letter from Sally explaining that Gabe has mysteriously vanished, her sculpture sold for enough money to start a new life, and Percy can either attend school in New York and live with her or stay at camp year-round.

On the Fourth of July, camp watches an elaborate Hephaestus-cabin fireworks show while Grover says goodbye before leaving on his secret search for Pan. Percy tries to trust that Grover will return, even though no searcher has come back in thousands of years. As July passes, Percy trains, worries over the Oracle’s final lines, and remains uneasy because the prophecy still does not feel fully resolved.

On the last day of summer, Percy receives Dionysus’s form letter forcing him to choose between staying at Camp Half-Blood or going home for the school year. While trying to clear his head in the sword arena, Percy finds Luke practicing with a new sword, Backbiter, which can harm both mortals and immortals. Luke invites Percy into the woods, offers him Cokes, and then reveals the prophecy’s betrayal: Luke serves Kronos. Luke explains that Kronos convinced him to steal Zeus’s master bolt and Hades’s helm, that Ares intercepted him and became an unwitting accomplice, and that Luke then manipulated events at camp so Percy would carry the stolen items toward Tartarus.

Luke admits summoning the hellhound and cursing the flying shoes, then leaves Percy to be killed by a pit scorpion rather than fight him directly. Percy kills the scorpion, but not before it stings him. Poisoned and nearly blind, Percy staggers back toward camp with help from woodland nymphs and collapses. In the Big House sickroom, Chiron and Annabeth revive him with nectar and healing. Percy tells them Luke is the true thief and that Kronos is behind the plot, but Chiron says Percy is not ready to pursue Luke yet and warns him not to let fear and anger make him easier for Kronos to influence.

Still weak, Percy goes outside with Annabeth and learns she has decided to return to her father for the school year after writing him a letter, following Percy’s earlier advice. The two promise to keep in touch and to go after Luke together if necessary. Watching Annabeth leave with her family and feeling camp suddenly empty, Percy finally makes his own decision: he will return to the mortal world, live with Sally, and come back to Camp Half-Blood next summer.