Annabeth offers a taxi driver LotusCash if he will drive them to Los Angeles. The taxi driver immediately speeds the young heroes West to Santa Monica. Percy sees the Pacific and marvels at the sea. He then meets the Nereid, a spirit of the sea, that spoke to him when he jumped off the Gateway Arch. She offers him three pearls that will allow him to escape the underworld.

Percy and his friends proceed, but when they are attacked by mortal thugs, they are forced to escape into a store. The owner, Crusty, approaches and tries to sell them beds. Crusty’s beds are made with great skill, but when Grover and Annabeth lie down on them, Crusty says “ergo” and the beds turn into traps. Percy realizes this is Procrustes; he tricks the salesman into demonstrating how comfortable his bed is by lying down in one. Percy says “ergo” and traps Procrustes before slaying him. They find a book of yellow pages for the Greek gods and monsters and use it to find the underworld. They make bribe Charon to let them enter the underworld. Then they face Cerberus. Before the three-headed dog attacks, Annabeth throws a red rubber ball and begins her “obedience school.” With Cerberus subdued, they proceed into the underworld.

Before they go far, Grover’s magical shoes begin to fly him toward a distant pit. Percy recognizes it as the pit he has seen in his dreams. He can feel evil emanating from the pit; this is something even older than the gods. There is a voice chanting, and Percy thinks it sounds like magic. Before Grover is taken to his death, he manages to slip his hooves out of the shoes. Percy now leads his friends on, though he notices that his backpack suddenly feels heavier.

When Percy meets Hades, the lord of the underworld, he is struck by the god’s power. However, he manages to stay calm and asks Hades to return Zeus’s master bolt, which enrages the god. Hades explains that he is actually missing his helm of darkness, which he thinks that Percy stole. Now he demands that Percy return the helm and surrender the master bolt, which has appeared in Percy’s backpack. In return, Hades will return Percy’s mother. Percy realizes they have been tricked. There are only three pearls, so Percy decides to use the three magic pearls to escape the underworld with Annabeth and Grover—he will find another way to rescue his mother. When they return to the surface, Percy discovers that Los Angeles has been wracked by a monstrous earthquake. The armies of Hades are invading.