Hank and Chuck, the bronze angel statues, fly Percy, Thalia, Zoë, and Grover over the Sierra Nevada toward San Francisco. Thalia keeps her eyes shut because of the height, while Percy tells her about Rachel Elizabeth Dare seeing through the Mist. Thalia explains that some mortals can do this, prompting Percy to realize his mother may have always seen monsters clearly.

The group lands at the San Francisco waterfront with no clear way to find Artemis, Annabeth, or the monster Artemis was hunting. Grover reminds Percy that Apollo told him to seek Nereus, the Old Man of the Sea. Zoë helps Percy disguise himself as a homeless person so Percy can approach Nereus unnoticed among the people near the docks.

Percy identifies Nereus by his foul ocean smell and tackles him. Nereus fights, dives into the bay, and changes forms into a seal, killer whale, eel, and finally a man, but Percy’s strength in the water lets Percy hold on. Nereus agrees to answer one question, and Percy chooses to ask where to find the monster that can destroy the gods. Nereus points at the water and escapes, revealing that the friendly cow serpent Percy had called Bessie is actually the Ophiotaurus.

Grover translates the Ophiotaurus’s speech, and Zoë recognizes the creature from an ancient Titan War prophecy: whoever kills the innocent Ophiotaurus and burns its entrails gains the power to destroy the gods. Thalia is visibly shaken by the temptation of that power. Dr. Thorn then ambushes the group with mortal mercenaries and reveals that Kronos intends Thalia, a child of the Big Three nearing her sixteenth birthday, to sacrifice the Ophiotaurus and overthrow Olympus.

Grover helps Bessie dive to safety, and the heroes flee through the pier, but Thorn’s forces surround them. Percy uses a broken water fountain and crystals to make an Iris-message to Camp Half-Blood, reaching Dionysus instead of Chiron. After Percy reluctantly asks for help, Dionysus drives the mercenaries mad and destroys the manticore with grapevines, then warns that Percy has only gained a few hours. Dionysus says Zoë knows where to go, and Zoë points across the bay toward the mountain and says she must return to the garden of her sisters.