Once Percy and his friends leave the casino, they get into a taxi and request it take them to the Santa Monica pier in Los Angeles. The driver tells them they must pay up front and they do not have any cash, but then Annabeth thinks to use the Lotus Casino debit card, which shows up with unlimited cash. In the car, they discuss Percy's dream, trying to figure out if the being in the pit is Hades or not. Percy says it did not feel like a god's voice. He remarks that the thing in the pit said it was waiting for two items—they deduce that one must be the master bolt, but do not know what the second is. Annabeth appears to have an idea of who might be in the pit, but refuses to tell them.
They make it to the pier, and Percy's first instinct is to go in the water. A shark appears, and he rides it deeper into the ocean. A woman materializes in front of him, and introduces herself as a Nereid, a spirit of the sea, who serves in Poseidon's court. She warns him about journeying to the Land of the Dead, and explains that few have done it and survived. She gives him a set of pearls and tells him to smash one if he is ever in need, and then tells him that everything belonging to the sea will ultimately return to it.
Next they head into Hollywood, looking for the Underworld address slip they had taken from the garden gnome emporium. They are approached by a gang of kids up to no good, and Percy instinctively tries to fight them off with Riptide, which does not work on mortals. To escape them, they duck into Crusty's Water Bed Palace, where a strange man named Crusty attempts to sell them a water bed. Ropes spring out and tie Grover and Annabeth to the bed, attempting to stretch them out to six feet so they fill the length of the beds. They realize this man is Procrustes, the giant in Greek mythology who had tried to killed the hero Theseus in a similar way. Percy manages to trick Procrustes into lying on one of the beds himself, and the ropes tie him up too. He slices him with Riptide as well for good measure.
They arrive at a recording studio building where the entrance to the Underworld is found. They encounter Charon, the ferryman to the Underworld, sitting at the security guard's desk. They ask to go to the Underworld and try to pretend they are dead, but he does not fall for it for long. Percy offers him a lot of drachmas to allow them through, saying Charon deserves more pay for his good work. He finally agrees, and they head off down the elevator which transforms to a boat across the River Styx.
Annabeth explains how the Underworld works: the fields of torture, the Asphodel Fields for the average person, and the Fields of Elysium for the especially good lives. Charon takes them all the way to Cerberus, the three-headed dog who guards the entrance to the Underworld. They are unsure how to get past him, but Annabeth begins to treat him like a regular dog, offering him a red rubber ball if he obeys her. This distracts him long enough for them to make it through the EZ DEATH line straight to the Asphodel Fields, though Cerberus is left behind longing for Annabeth, his new friend.
The Fields of Asphodel are huge, and millions of spirits float aimlessly through them. They spot the Fields of Punishment, a volcanic wasteland, in the distance with Sisyphus moving a boulder up and down a hill for eternity. They also see a valley that holds Elysium, and in the middle, the Isles of the Blest, the place for heroes who had achieved Elysium three times. Percy thinks about how depressing it is that so few people have realized goodness in their lives.
Suddenly Grover's winged sneakers go out of control and carry him off, and Percy and Annabeth follow, alarmed. They lead them down a tunnel until they eventually reach the chasm that Percy has been seeing in his dreams. They think Grover is going to be dragged down into the pit until he hits a rock and his shoes fly off, freeing him. Annabeth realizes they are at the entrance to Tartarus, and they hear a muttering, evil voice from far below. They get out quickly back through the tunnel and head towards the palace of Hades instead to do what they need to do.
In the palace they encounter the garden of Persephone, Hades's wife who was taken from the upper world. The guards let them into the palace and Percy gets his first glimpse of Hades, the first god he has seen who appears godlike. Percy is immediately affected by Hades's powerful aura, reminiscent of leaders like Hitler or Napoleon. Hades allows Percy to speak, and he requests that Hades return the master bolt. Hades denies wanting a war, because his kingdom has become far too large, and denies stealing the master bolt. Hades accuses Percy of being the thief on the winter solstice, taking both the bolt and his Helm of Darkness. He tells Percy to return the helm now or he will stop death, opening the gates of the Underworld to allow the dead to pour back into earth and make it a nightmare.
Percy denies all of this, but Hades tells him to open his pack, and Percy realizes that the master bolt is in there, though he does not understand how. Hades demands the bolt and his helm as well, and shows Percy his mother, frozen in a shower of gold. Percy remembers the three pearls in his pocket from the Nereid, but Hades says that each will save only one person, so either one of them or his mother must be left behind. Grover and Annabeth both offer to stay behind, and Percy wants desperately to sacrifice himself for his mother, but he knows she would never allow it and remembers the words of the prophecy: "You will fail to save what matters most in the end."
Percy promises Hades he will find his helm and then each of the three of them smashes a pearl at their feet, then are taken back to the surface in milky white bubbles. They make it back up and realize Hades has sent an earthquake to shake Los Angeles, and it is the morning of the summer solstice. Percy has only two thoughts in mind: making it to Olympus to return Zeus's bolt, and having a conversation with the god who tricked him.
Just like Grover, Annabeth has proven herself to be much more than just a sidekick on this journey as well. As the daughter of the goddess of wisdom, she is naturally intelligent—however, she has also been part of the world of gods, demigods, and misters for longer than Percy has, so she has a lot more exposure to the myths, legends, and occurrences that govern it. Annabeth has a knack for figuring things out before everyone else, as evidenced by her hunch about the being in the pit, but she is logical and cautious enough to not make any claims until she is certain. Percy would not have made it this far without Annabeth's help, proving that heroes do not work alone.
The scene at Santa Monica connects Percy with his father more than any other scene to this point. Before he enters the ocean, he is in awe of the vastness of Poseidon's realm. Once he is under the water, his sense of comfort and belonging is undeniable, a true testament to his lineage. Part of Percy's development during this quest is learning to embrace his new identity as the demigod son of Poseidon. When he left Camp Half-Blood, he was uncomfortable with the idea of his parentage, but as time goes on he slowly begins to accept and even take pride in his father.
In Los Angeles, Percy faces tests of cleverness rather than tests of skill or strength. He manages to outsmart Crusty and get him to lie in one of his own beds, bringing about his downfall. He convinces Charon to allow them into the Underworld by charming him. In both cases, he recognizes what these monsters want most—validation—and uses this knowledge to his advantage in order to get what he wants.
As Percy and his friends begin to explore the Underworld, the interesting question of goodness versus evil comes up once again. This time, the answer is not as black and white as mythology makes it seem. Yes, there is Elysium for the good souls and the Fields of Punishment for the bad, but the vast majority of people fall somewhere in between, which is an important truth about the world that Percy must learn to accept. He also remarks on how few people have actually done good in the world, and seeing Elysium with his own eyes provides the motivation he needs to make sure that he lives up to expectations as a true hero and lives a life of goodness.
Hades is a complex character who also blurs the lines of good and evil. Until this point, Hades has been characterized as the evil god, the thief, the one two turned bad while his two siblings were allowed to stay and reign in Olympus. Once it is revealed that he is not the thief, though, it becomes unclear whether he truly is evil. The only characteristics of Hades's that are undeniable are his power and charisma that captivate everyone in his presence. He uses this to his advantage, but this does not necessarily make him evil, because many of the other gods—particularly Ares—are similarly manipulative. Hades is motivated by his desire to repossess what is his, which, in essence, is not an evil thing to want.
At the end of his time in the Underworld, Percy chooses to save himself and his friends rather than his mother, making that telling line of the prophecy come true. Contrary to what it may seem, this is an incredibly mature decision that reflects Percy's personal growth. At the beginning of his quest, Percy was blinded to all practicalities, thinking only of rescuing his mother rather than about duty and obligation. By choosing to save himself, he displays that he both understands and heeds what she would want—for him to live instead of her—and what is good for the world. If he were to die, then he would not be able to return the bolt, track down the helm, and prevent the largest war mankind has ever seen.