Summary:
* Percy is still reeling from the events of the previous chapters, unsure if they are real or just a hallucination.
* He returns to his cabin and finds a note telling him to be at the Half-Blood Hill dock at midnight.
* Percy finds Annabeth, Grover, and Luke waiting there. They tell him that they believe Poseidon is his father, and that he must travel to the Underworld to retrieve Zeus's stolen lightning bolt before the summer solstice, or war will break out between the gods.
* The group travels to New Jersey, where they enter the Underworld by taking a magic subway train.
* They encounter various monsters, including Cerberus, the three-headed dog who guards the gates of the Underworld.
* They finally meet Hades, god of the Underworld and father of Percy's rival Nico di Angelo, who accuses them of stealing the lightning bolt.
* Percy's group escapes from Hades' realm with help from the souls of the dead, including Luke's mother.