In the first scene of the novel, Percy and his fellow demigod Charles Beckendorf infiltrate Kronos’s ship. The latter is the Titan god of time, and he is leading the fight against the Olympians. Kronos has overtaken the body of Luke Castellan, a demigod who has bathed in the River Styx to become invulnerable. Although Percy and Charles succeed in blowing up the ship, Charles dies before he escapes.
Percy returns to Camp Half-Blood, where he encounters his mentor Chiron (a centaur) and love interest Annabeth Chase (daughter of Athena). The Oracle of Delphi, in the form of a mummified doll, reveals the Great Prophecy—the hero will make a choice to ensure the ultimate success or destruction of Olympus, a choice which will ensure the hero’s demise. Although it appears as though Percy is the hero mentioned in the prophecy, the author later reveals that it is Luke.
Once deciding to accept the Prophecy, Percy goes on a journey with Nico di Angelo, son of Hades, to learn more about Luke’s background in order to better be able to defeat him. They visit May Castellan, Luke’s mother, who has lost touch with reality. She is a mortal but has the gift of prophecy and tried to take on the role of the new Oracle of Delphi but was cursed.