Percy dreams of Mount Othrys, where Titans discuss the destruction of the Princess Andromeda. A golden Titan dismisses the explosion as unimportant, orders Krios to guard Atlas, and says the gods are already occupied by a first challenge while more threats are coming. Nico di Angelo appears beside Percy in the dream and warns that Percy is running out of time unless he accepts Nico's dangerous plan.

Percy wakes underwater in Poseidon's palace, where Tyson has been watching over him after finding him sinking through the sea. Tyson confirms that the Princess Andromeda exploded and says there is no sign of Beckendorf. Percy realizes Beckendorf almost certainly died in the blast and feels guilty for surviving and leaving him behind.

Tyson brings Percy through Poseidon's realm, which is magnificent but under siege. Percy sees wounded mermen, ruined defenses, Greek fire burning underwater, and huge monsters attacking the palace. Poseidon destroys a giant squid from a rooftop command center, where Percy also meets Amphitrite, Triton, and Delphin, and notices the strained reactions within Poseidon's immortal family.

Poseidon appears aged and exhausted because his realm is suffering. He explains that Oceanus, the Titan of the sea, has joined Kronos after staying neutral in the first Titan war, which means Oceanus believes Kronos may win. Percy also sees Briares fighting among Poseidon's forces, but Poseidon admits the war has drained him and that Oceanus continues sending ancient sea monsters against them.

Percy reports the mission against Kronos, grieving Beckendorf and the demigods who may have died on the ship. Poseidon tells Percy that Beckendorf chose a heroic death and that the sabotage scattered Kronos's invasion force, buying New York time. However, Poseidon refuses Percy's offer to stay and fight, saying Percy's battle is above and that Percy must return to Camp Half-Blood, warn his friends about the spy, and hear the full Great Prophecy from Chiron.

Tyson is sent back to the armory despite wanting to fight, and Percy urges Poseidon to trust him more. As Oceanus approaches, Poseidon prepares to assume his true godly form and reminds Percy to keep the sand dollar birthday gift until the right time. Percy leaves for the surface while Poseidon faces Oceanus and the sea erupts in green and blue flashes of battle.