Percy Jackson spends a rare peaceful August afternoon driving along a South Shore beach with Rachel Elizabeth Dare. Rachel invites Percy to join her family in St. Thomas, partly because she wants relief from her father’s pressure to attend Clarion Ladies Academy and partly because she has something serious to discuss. Percy wants to go, but the war with Kronos and the prophecy surrounding Percy’s upcoming sixteenth birthday make him uncertain.

Before Rachel can fully explain, Blackjack the pegasus lands on Paul Blofis’s Prius with Charles Beckendorf aboard. Beckendorf has arrived for a planned mission against the Princess Andromeda, Kronos’s cruise-ship headquarters. Rachel kisses Percy goodbye, promises to explain the damaged car, and Percy leaves with Beckendorf, wondering whether Percy will survive long enough to learn what Rachel meant to say.

At dusk, Blackjack drops Percy and Beckendorf on the Princess Andromeda, which is heading for New York Harbor. Percy and Beckendorf infiltrate the ship’s lower decks and reach the engine room, where Beckendorf begins setting Greek fire explosives. When approaching monsters threaten to discover them, Percy leaves Beckendorf to finish arming the charges while Percy creates a diversion.

Percy battles and evades monsters throughout the ship, including telkhines, dracaenae, and a giant crab in the promenade. Percy kills the crab by using water from a fountain to slide beneath it and stab the weak spot in its belly. Percy also spares a young demigod serving Kronos, telling the boy to escape and warn the others, because Percy sees him as another child being manipulated by the Titan’s war.

On the upper deck, Percy discovers the diversion has led him into a trap. Kronos, possessing Luke Castellan’s body, reveals that Kronos expected Percy because of a spy at Camp Half-Blood. Percy attacks, but Kronos slows time and wounds Percy with a scythe that drains more than blood. Salt water from the pool breaks the time spell long enough for Percy to fight back, but Percy cannot defeat Kronos, whose body seems unnaturally invulnerable.

Kronos’s forces drag in Beckendorf, badly beaten, and Ethan Nakamura reports the capture. Percy realizes Beckendorf deceived the monsters into thinking the explosives were not set, while the peach cans in Beckendorf’s bag expose the trick. Beckendorf silently signals that the detonator has no delay and urges Percy to go. When Beckendorf reaches for the watch detonator, Percy distracts Kronos and jumps into the sea; moments later, Beckendorf triggers the explosives, and the Princess Andromeda erupts in green fire as Percy blacks out underwater.