As the quest group crosses the Potomac, the same black helicopter seen at Westover Hall comes after their van. Percy realizes the enemy knows the vehicle, and Bianca directs Zoë into a mall parking lot with a subway entrance, allowing the group to abandon the van and escape south by train.

On the subway, Bianca says she remembers living in D.C. with Nico when there was no subway at all, even though the station looks old. The strange comment unsettles the others, but the helicopter’s continued search forces them to focus on changing trains until they lose it.

The group ends up in a cold industrial rail yard with no clear route west. A grimy homeless man offers them warmth and points out a snow-free automobile freight train labeled Sun West Line; when the group turns back, the man and his fire have vanished, suggesting divine help. Soon they are riding west in luxury cars stored on the freight train.

During the ride, Percy joins Thalia in a Mercedes. Thalia explains they are likely headed for San Francisco, where Artemis had gone and where Titan magic near the Mountain of Despair makes the Mist thick and monsters especially dangerous. Percy deduces that Thalia and Zoë clash because the Hunters once tried to recruit Thalia; Thalia admits she nearly joined but refused to leave Luke, and she becomes upset when Percy asks whether she could kill Luke if necessary. Before shutting Percy out, Thalia adds that Annabeth also wanted to join the Hunters and tells Percy to consider why.

Later, Percy sits in Grover’s Lamborghini while Grover sleeps. The homeless man appears beside Percy and reveals himself as Apollo in disguise, saying he has helped by speeding the train west but cannot openly interfere further. Apollo does not know where Artemis is because she is hidden from him, but he advises Percy to seek Nereus, the Old Man of the Sea, in San Francisco if the group still has not found the monster.

Apollo puts Percy to sleep, and Percy dreams he is an ancient warrior wearing the Nemean Lion’s skin in a starry garden. A frightened girl, later recognized as Zoë Nightshade, urges the warrior to avoid Ladon and trick her father, then gives him a hairpin containing ocean power; it transforms into the bronze sword Anaklusmos, Riptide. Percy wakes as the train stops in snowy mountains and realizes Zoë once gave his sword to its original wielder.