Chiron gives Percy and his friends some nectar and ambrosia, food of the gods, to use if they need healing on the trip. Annabeth brings her magic Yankees cap, which makes her invisible. Luke gives them a gift of winged sneakers, but since Percy cannot be in the air because Zeus controls it, he gives them to Grover instead. Finally, Chiron gives Percy the pen sword, a gift from Poseidon; its name is Anaklusmos, or Riptide. It can only harm monsters, not mortals, and it will always reappear in his pocket, so he cannot lose it. Mortals will not be able to see it because of something called the mist, which obscures mythological happenings. All packed, Argus, the one hundred-eyed security guard, drives them to catch their bus.
In the city, Percy spots "Missing Boy" signs up for him, and he begins thinking about Gabe and their apartment. Since satyrs can read minds, Grover guesses what Percy is thinking, and explains that his mother married Gabe because his mortal stench was so strong that she hoped it would obscure Percy's demigod scent from monsters. Percy starts thinking about getting his mother back from Hades when they go to the underworld.
They get on the bus and watch three old ladies get on behind them, and Percy notices that one of them is Mrs. Dodds. The three Furies are on the bus with them. Percy puts on Annabeth's invisibility cap and sneaks to the front of the bus, but then the Furies transform and begin a fight. Percy hits the emergency brake and stops the bus, and then manages to slice the Furies except Mrs. Dodds and make them disintegrate. Percy, Annabeth, and Grover run quickly away from the bus, leaving all their money, food, and clothes behind.
They make it to a roadside curb shop that sells garden gnomes and various other garden statues called "Aunty Em's Garden Gnome Emporium." It smells like hamburgers and they are starving, so they head towards it despite Grover's warnings to stay away. Grover spots a satyr statue that looks like his uncle. They are greeted by a woman wearing a long black gown and veil, and when she asks what they are doing there alone, Percy lies and says they are orphans who have lost their circus caravan. This woman, who calls herself Aunty Em, offers to make them food for free. Percy does not notice when Aunty Em calls Annabeth by her name, even though they never introduced themselves.
Aunty Em starts talking about making all the statues, and Annabeth starts to catch on to who she really is. Aunty Em tries to get them to stay to pose for a picture, but Annabeth tries to rush them out, realizing that Aunty Em is Medusa and she has turned all these creatures into stone. It is confirmed when they see the snakes on her head. Medusa is especially out to get Annabeth, because it was Athena who made her like this. After a long fight, Percy manages to slice Medusa's head off the way his namesake, Perseus, did in the original myth. They wrap it up and put it in a box addressed to the gods on Mount Olympus and send it by Hermes Overnight Express.
That night, Grover explains to Percy what he means when he says he wants to get his searcher's license. Searchers are satyrs who go in search of Pan, the God of Wild Places; none of them has returned alive. Grover wants to be the first to succeed. Percy says the only reason he wants to go to the Underworld is to save his mother, but Grover reads his emotions and says he wants to make his dad proud, too. Percy sleeps and has a dream about a cavern with a strange being inside of it, asking him to bring him the bolt. He wakes to find Grover talking to a lost pink poodle, Gladiola, who agrees to be returned to her family so that the questers can receive the family's $200 reward and have money to buy a train ticket to Los Angeles.
They spend two days on the train, where Percy reads newspapers and learns that he is now a wanted fugitive for the bus incident. Annabeth tells him about her mortal father, who never wanted her because he was too caught up in his mortal family, causing her to run away. She also reveals that she wants to be an architect. She and Percy have been constantly fighting, but they agree to cooperate the way Athena and Poseidon did when they made the chariot.
They make it to St. Louis and go sightseeing at the Gateway Arch before catching the next train west. They talk about Hades's Helm of Darkness, which allows him to become the darkness itself, melting into shadow. A lady and her chihuahua get into the elevator car with them to go up the arch, and Percy notes that the dog has a forked tongue. Annabeth and Grover go down in the elevator before him because he cannot fit, and suddenly the dog transforms into a chimera and the woman reveals herself to be the Mother of Monsters, Echidna. The chimera breathes fire and blows a hole in the side of the arch, and Percy tries to attack him with Riptide and fails. The sword falls out the hole in the arch and into the river and does not immediately return to him, and Echidna dares him to jump and prove his lineage, because if he is truly Poseidon's son he will survive. He jumps.
As Percy leaves Camp Half-Blood, the one place specifically designated for demigods like him, he begins to feel the struggle of the in-between. As a half-god, half-human, he feels like he cannot fit in anywhere; not the mortal world, nor the divine world of his father. Demigods like Percy sit at the crossroads of two different worlds, and must deal with the challenges associated with both of them—things like school on one side, and monsters on the other. This struggle to find a place and feeling in-between is a common one for literary heroes.
The first battle on the bus from New York City sets up the series of tests that the hero must confront on his quest. In epic Greek hero stories like The Odyssey, these tests are very often physical challenges of the hero's strength and quick wit, and in this first test, Percy rises to the occasion. Later on, though, it becomes clear that he cannot surmount every obstacle on his own. He needs his friends' help to recognize Medusa for who she is and save them, even though he is the one who ultimately defeats her.
Grover and Annabeth are considered Percy's "sidekicks," but really, they have stories and aims of their own that keep them motivated throughout their quest. Annabeth details the story of her life spent feeling unwanted by her mortal father, and this quest is a chance for her to prove herself. Grover has to prove himself, too, in order to get the searcher's license and embark on what he believes to be his true purpose in life: a quest to find the god Pan. It is important to remember that these two are complex characters who are on this quest not solely to help Percy, but also to achieve their own goals.
As the journey goes on, the book includes more and more allusions to Greek mythological stories. Readers are not expected to know all of them before reading, and one of the key features of this series is that the books aim to teach young readers these important stories in an engaging, relatable way. Because Percy needs to learn a lot of them, too, readers can learn about them through Annabeth's exposition to him. The strongest allusion in this section is to the original Perseus fighting Medusa, but Annabeth also mentions the story of Athena and Poseidon cooperating to build a chariot, a combination of both of their skills.
Chapter 13 ends on a cliffhanger, with Percy jumping into the river from the Gateway Arch after being attacked by the chimera. This is an important moment, because it represents the first time Percy has openly trusted his father to come to his aid in a time of trouble. Only a few days before, Percy had said to Grover that his only motivation on this trip is to go to the underworld and bring back his mother, but Grover saw through that ruse and knew Percy was, in part, doing it out of pride in his father. This incident proves that this is true: Percy places an immense amount of faith in his father by jumping off the arch, showing that he really does value the god's role in his life.