Percy Jackson is a twelve-year-old boy from New York City, and up until a few months ago he was a boarding student at Yancy Academy, a private boarding school for troubled kids in upstate New York. He says he didn't want to be a half-blood, and advises any readers who think they might be one to close this book immediately. He backs up to tell the story of how it all started.
He and his sixth-grade class at Yancy go on a field trip to Manhattan to the Metropolitan Museum of Art, planning to see ancient Greek and Roman artifacts. Mr. Brunner, their Latin teacher whom Percy really likes, is leading the trip. Percy hopes he will not get in trouble on this trip, but he starts to get mad when Nancy Bobofit throws bits of peanut butter at his best friend Grover. Grover warns him not to do anything, because he knows if anything happens, Percy will get blamed, as usual.
Mr. Brunner grills Percy on the story of the Titan Kronos who ate his children, and tells Percy he must start thinking about how important this information is in his real life. Percy is frustrated, and when Nancy Bobofit continues to torment Grover, she suddenly finds herself sitting in the fountain, claiming Percy pushed her. Percy does not know what happened, but he knows he is in trouble.
Mrs. Dodds, the math teacher, tells Percy to come with her. She starts telling Percy to confess, but he has no idea what she is talking about. Suddenly she turns into a strange creature with leathery wings. Mr. Brunner comes in and throws a pen at Percy, and as soon as it hits his hand, it turns into a sword. Mrs. Dodds flies at him, shouting for him to die, but as soon as Percy's blade hits her shoulder she explodes into yellow powder and disappears. Percy is stunned, and goes back to his class—his classmates, however, claim that they have never had a Mrs. Dodds as a teacher and they have no idea what he is talking about.
Back at school, the students continue to deny Mrs. Dodds' existence. Percy knows something has happened, and it makes him irritable, so his grades begin to slip and he gets in trouble often. The headmaster sends his mother a letter saying he will not be invited back to Yancy Academy. Percy hears Mr. Brunner having a conversation with Grover about Mrs. Dodds and being worried about Percy, and he realizes something is happening behind his back.
Percy and Grover head back to Manhattan on the same Greyhound bus. Percy tells Grover he overheard him talking to Mr. Brunner, and Grover tells Percy that he has been trying to protect him. Before he can explain, the bus breaks down, and on the side of the road Percy sees three old ladies knitting socks. He watches one of them take out her scissors and cut the yarn. When they get back on the bus, Grover asks Percy what he saw, and freaks out when Percy tells him. Percy guesses that the snipping of the yarn means someone is going to die.
Percy ditches Grover as soon as they get to the bus terminal in New York and heads uptown to his apartment. He tells readers about his mother, Sally Jackson, an amazing woman who has unfortunately had some terrible luck—the one good break she got was meeting Percy's dad, of whom Percy himself has no memories. They were not married, and one day he left to sail out on some journey and never came back. She married an awful man named Gabe Ugliano, who Percy calls Smelly Gabe. Gabe is rude to Percy when he gets back to the apartment, telling him his report card came. Percy feels better about everything when his mother comes home from her job at the candy store in Grand Central Station, giving him a big hug.
She tells Percy that they are going out to Montauk, to the beach cabin where they used to go every summer. Gabe at first refuses to let them go, but eventually allows them to take his car as long as they do not let anything happen to it. They drive out to Montauk, and both Percy and his mother are extremely happy to be there—it was where his mother met his father. She tells Percy his father would be so proud of him, and they talk about him. His mother reveals that she will have to send Percy away to a summer camp keep him safe, because it is not safe for him to be close to her anymore. She does not explain further.
They awake in the middle of the night to a hurricane. There is pounding on their cabin door and it is Grover, and Percy is shocked to see that he is not wearing pants. Instead, where his feet should be are cloven hooves. Grover tells them they need to get away immediately, and they jump in Gabe's car.
The novel begins with Percy, the protagonist, breaking the fourth wall and speaking directly to readers in a colloquial, relatable tone. This technique draws readers in, making them feel as if they are an important part of the world Percy lives in. This choice to speak to readers—even suggesting that some readers might be half-bloods themselves—makes reading this story an extremely immersive experience, which is important for a young adult fantasy book like this.
Since Percy is telling this story from some time in the future, looking back on his experience, these first few chapters use a lot of foreshadowing to draw readers in. Percy immediately talks about how he did not want all of this to happen to him, and about how being a half-blood is terrible. Readers do not know yet exactly what he went through, but they do know that it was incredibly challenging and frightening, and this foreshadowing of dramatic events to come hooks them immediately. Foreshadowing continues with Percy sighting the three old ladies cutting the yarn; this is an allusion to the mythological Fates. A person who sees the Fates cut the thread of life is doomed to die, and this sighting foreshadows trouble for Percy.
These first chapters also set Percy Jackson up to fit the typical epic hero mold outlined by his classical predecessors, like Odysseus from Homer's The Odyssey. Like other epic heroes, Percy is an outsider, not fitting in with his peers and being pushed away by those around him because he is different in some way. Also like other heroes, he knows very little about his past; though his mother is present in his life, his father is not, and absent parents are a hallmark trait of epic heroes. Percy Jackson is immediately established as a modern version of the classical heroes, and even though readers do not yet know what makes him special, they are already set up to associate him with these ancient figures.
Mr. Brunner continues the foreshadowing by hinting that Greek mythology is important to Percy's life in some way, even though Percy does not yet understand how yet. He singles Percy out and treats him differently from the rest of the students, suggesting that there is something special about him. That this conversation happens just before Mrs. Dodds attacks Percy is no coincidence: readers can line up these two occurrences and make inferences about who Percy really is and why strange things have been happening to him.
Because everyone is trying to protect Percy, it appears that the other significant people in Percy's life—Grover, Mr. Brunner, his mother—all know a secret about him that he does not yet know. Percy is characterized as stubborn and independent, refusing to let everyone hover over him in this way. Despite their best efforts, though, these people are unable to protect Percy for long before his true identity finds him, which goes along with the novel's theme of fate and unavoidable destiny.
A common motif throughout these chapters is the water and the ocean, and these symbols become increasingly important when Percy and his mother take their trip out to Montauk. Both Percy and his mother associate the sea with his father, and this connection is the first of many clues that will lead readers to guess who his father really is.