Twelve-year-old Percy Jackson warns the reader that if they recognize themselves in the story to follow, they should stop reading: being a demigod is rough. His story begins with a school field trip to the Metropolitan Museum of Art. Percy has ADHD and dyslexia, and he’s gotten expelled from six schools already. He tries hard to be good because he admires his Latin teacher, Mr. Brunner, but the evil math teacher Mrs. Dodds ruins everything. She takes Percy into the museum alone and transforms into a monstrous creature. Mr. Brunner appears and tosses Percy a pen that turns into a sword, and Percy slays Mrs. Dodds. Everything goes back to normal—but other students believe that someone named Mrs. Kerr is the math teacher. Only Percy’s best friend, Grover, seems aware that Mrs. Dodds was real, but he won’t confirm this. Percy’s grades slip, and he gets expelled at the end of the year.
As Percy and Grover ride the Greyhound bus home, the bus breaks down. Percy and Grover see a roadside fruit stand where three old ladies are knitting and cutting thread. This freaks Grover out, so he asks to walk Percy to his apartment. Percy ditches Grover, evades Smelly Gabe (Mom’s horrendous husband), and waits for Mom. Mom announces that she and Percy are going to Montauk for the weekend without Gabe. The vacation starts out normal—they talk about Percy’s father, who left when Mom was pregnant—but a hurricane rolls in. Grover shows up, but he has the lower body of a goat and says that “he” is after them. Mom tries to drive Percy to a summer camp where he’ll be safe, but lightning strikes the car and a Minotaur attacks them. Percy and Mom try to get Grover, who’s injured, past a huge pine tree, but the Minotaur snatches Mom and she dissolves. Percy manages to slay the Minotaur before passing out. When he wakes, Grover takes Percy to meet Mr. D, the camp director. Mr. D is with Mr. Brunner, but Mr. Brunner says that isn’t his real name—he’s the centaur Chiron. Mr. D reveals that he’s Dionysus, the god of wine. Chiron explains that the Greek gods are real; Mount Olympus is in New York City. Chiron and a girl named Annabeth take Percy on a tour of Camp Half-Blood, where the campers are all demigods (children of one god parent and one mortal parent). At the Hermes cabin, Percy meets his counselor, Luke. Later, one of Ares’s daughters, Clarisse, tries to shove Percy’s face in a toilet, but he somehow makes the water shoot back into her face. Percy is shocked to discover that his father is one of the Greek gods—but his father might never claim him. Until Percy’s father steps up, he’ll have to stay in the Hermes cabin. Annabeth and Percy discuss what’s going on. The weather seems like the gods are fighting, and it’s been this way since the winter solstice. Annabeth believes that something was stolen, and she desperately wants to go on a quest to find it.
Percy falls into a routine at camp, but he focuses on the possibility that Mom is imprisoned in the Underworld. He also begins to understand why some kids, like Luke, resent their immortal parents. Several odd things happen: Percy performs a tricky sword maneuver after dumping water on himself, and he’s exceptionally good at canoeing. One afternoon, Grover shares that his assignment to protect Percy isn’t over yet. He also tells Percy why there are no children of Zeus, Poseidon, and Hades at camp: they promised after World War II to not father more children. Zeus messed up 17 years ago and had a daughter named Thalia, but monsters killed her. Later, the camp plays capture the flag. Percy has to stand guard by a stream. Clarisse and other Ares kids accost Percy, but when he falls into the water, he feels strong. As he fights them, his team wins. Immediately after, a hellhound attempts to kill Percy, and a trident appears above Percy’s head: this means that his father is Poseidon.
The next morning, after a nightmare of two men fighting, Percy receives a quest. Chiron shares that Poseidon and Zeus are fighting, as Zeus’s thunderbolt is missing—and he believes Percy took it. Percy speaks to the Oracle, which says that Percy will go west to face a god who turned, that he’ll find and return what was stolen, that a friend will betray him, and that he’ll fail at what matters most. Chiron says that Percy must journey to Los Angeles and the Underworld because Hades has the thunderbolt. Percy is thrilled because this means he can rescue Mom. Grover and Annabeth offer to go too. Before Percy leaves, Luke gives him a pair of winged shoes that Percy passes on to Grover; flying is dangerous for Percy since Zeus hates him. Chiron gives Percy his pen, which transforms into a sword called Riptide. On the way to the bus, Annabeth insists that she and Percy can’t be friends because Poseidon and Annabeth’s mother, Athena, are rivals. On the bus, the Furies attack and ask where “it” is. Percy, Annabeth, and Grover manage to escape the bus and hike until they reach a business called Aunty Em’s Garden Gnome Emporium. Grover insists that he smells monsters, but Annabeth and Percy are too intent on the smell of burgers. Aunty Em, the owner, is dressed in a long gown and veil that covers her face. She makes them food. As Percy grows sleepy, Aunty Em asks to take their photo. Percy agrees, but he realizes that Aunty Em is actually Medusa—her garden statues are petrified people. Percy slays Medusa and ships her head to Mount Olympus.
That night, Grover confides in Percy that he wants to be a searcher and spend his life searching for the god Pan. They discuss that the Furies are looking for an object, not Percy, but Percy doesn’t care—he just wants to save Mom. Percy dreams of a voice in a terrible pit that tries to use Percy to drag itself out. The next morning, they secure train tickets to Denver. On a layover in St. Louis, Annabeth insists on visiting the Gateway Arch. Percy thinks it feels wrong, but he rides the elevator up to the top with a woman and her Chihuahua. After Grover and Annabeth head back down, the woman turns into Echidna and the Chihuahua turns into the Chimera. Percy dives off the monument into the Mississippi River, where he receives a message: he must go to Santa Monica beach before going into the Underworld, and he mustn’t trust the gifts.
The trio arrives in Denver with seven days to complete their quest. They decide to call Half-Blood Hill via an Iris message. Percy speaks to Luke about his dreams and his concerns that he’s on the wrong track, but Luke insists that Hades stole the bolt and mentions something odd about no one turning into a pine tree. After this, the kids go to a diner, where a huge biker walks in and offers to pay for their meal. The man is Ares, the god of war. He needs Percy to do him a favor and fetch his shield from a waterpark in exchange for transport to L.A. Percy only agrees when Ares says he has information on Mom. At the waterpark, Percy is shocked to discover that Ares is dating Aphrodite even though she’s married to Hephaestus. Percy discovers Ares’s shield in a ride, but as he touches the shield, cameras appear to broadcast live to Mount Olympus. Percy manages to procure water and save himself and Annabeth. He’s incensed that Ares tricked them, but he accepts Ares’ offer of a backpack with food and clothes. Ares also shares that Mom isn’t dead.
Percy, Annabeth, and Grover get into a truck transporting three abused zoo animals. Percy shares what Luke said about the pine tree, and Grover and Annabeth elaborate: Grover was Thalia’s keeper, and Luke and Annabeth joined them on their way to Camp Half-Blood. Monsters attacked, and Thalia sacrificed herself—but Zeus turned her into the pine tree at the top of Half-Blood Hill. After Grover falls asleep, Percy asks if Athena and Poseidon will fight each other again, like they did in the Trojan War. Annabeth says that no matter what, she’s fighting alongside Percy. Percy has another dream about the voice in the pit, and he recognizes a second voice. The next day, Percy lets the animals go in Vegas and the kids enter a hotel. They play games until Percy meets kids who’ve been hypnotized and stuck in the hotel for decades. When Percy drags Annabeth and Grover out, they discover they have one day to complete their quest and that Percy no longer remembers his dream. They get a taxi to L.A., where Percy wades out into the ocean. A sea spirit, Nereid, gives Percy three pearls to smash if he needs help.
On their way to the recording studio obscuring the Underworld, the trio gets caught by a man named Crusty—the monster Procrustes, who stretches or cuts people to fit on six-foot-long mattresses. In the lobby for the Underworld, Mr. Charon refuses to take Percy to the Underworld, but he agrees when Percy promises to tell Hades to give Mr. Charon a raise. Annabeth gets them past Cerberus, the three-headed dog, by playing ball with him. Percy leads his friends straight to Hades’s palace, but Grover’s shoes sprout wings and carry him away, straight toward the pit from Percy’s dreams. They escape because the shoes fit poorly around Grover’s hooves. Finally, they enter the palace. Hades is incensed about what that Percy did, while Percy asks for the bolt back and begs Hades to not start a war. Hades insists that he doesn’t want a war—he has too many dead people already. He admits that his magical object, the helm of darkness, is missing. He believes Percy has the helm and the bolt, so he conjures Mom and threatens to kill her if Percy doesn’t hand the objects over. Percy discovers the bolt in the backpack that Ares gave him. Annabeth and Grover offer to stay in the Underworld in Mom’s place, but Percy knows that Mom wouldn’t forgive him if he allowed that. He gives his friends the pearls, and the three of them escape through the ocean. On the beach, Ares reveals that he set Percy up with the bolt: he wants a war, and he also has the helm. Percy challenges Ares to a fight. He believes he can win because he suspects that the being in the pit is controlling Ares. Percy injures Ares, and a dark force passing through ends the fight. Reporters and police at the scene “rescue” Percy and raise money to fly them back to New York, while the Furies take Hades’s helm back to him.
Percy goes alone to the Empire State Building. When he reaches the throne room in Olympus, he bows to Poseidon. Poseidon seems distant, which Percy thinks is the most honest reaction. Percy returns Zeus’s thunderbolt and tells Zeus everything, including his suspicions about what’s in the pit in the Underworld. Zeus disappears. Poseidon confirms that Kronos, his and Zeus’s father, lives in the pit, and he tries to convince Percy that Kronos is powerless. He tells Percy that Mom is home now. As Percy leaves, Poseidon praises Percy. Percy then reunites with Mom. Gabe seems even crueler than ever, and Percy realizes that he’s been hitting Mom. In his bedroom, Percy finds the box containing Medusa’s head. He tells Mom what’s in the box and returns to camp, feeling as though he belongs. He bids Grover goodbye when Grover leaves to search, and he’s thrilled when he learns that Mom sold her first sculpture (the petrified Gabe) and is using the money to pay for college. She offers to let Percy live at home to attend school.
The last morning of camp rolls around, and Percy hasn’t decided what to do. He agrees to go find monsters with Luke. In the woods, however, Luke reveals that he’s a double agent: he’s working for Kronos, and he stole the bolt and the helm. He sets a pit scorpion on Percy. Percy manages to defeat the scorpion, after which he goes to the farmhouse. Chiron convinces him that he can’t do anything about Luke or Kronos now, but he does say that Percy will be a great hero. Percy decides to go home for the year when Annabeth shares that she’s going to try to live at home with her dad. Percy promises Poseidon that he’ll return next summer.
