In this chapter, Percy and his friends travel to the Underworld, where they meet with Nico di Angelo. Nico tells them about the prophecy that states that Percy will either defeat Kronos or die trying, and that the way to defeat Kronos is to find and bathe in the River Styx, a poisonous river that flows in the Underworld.
Percy, Annabeth, and Grover decide to go to the River Styx, but Nico warns them that it is a dangerous journey. They go through the Fields of Asphodel and meet many spirits that have died. They face several obstacles, such as being attacked by Furies and Stymphalian birds.
Eventually, they reach the River Styx and decide to bathe in its waters. Percy successfully emerges from the river without being harmed, but Annabeth and Grover pass through a portal and are pulled into the sky above Olympus.