Percy dreams about Grover being chased by a monster. The satyr says he has to warn them and then Percy wakes up, seeing a shadow outside his window. He tells his mom about the dream. She offers to take Percy to a skateboard shop after his last day of school, but he asks about packing for camp. She says she got a letter from Chiron telling them to post pone Percy’s arrival because it isn’t safe. She promises to explain later after school.
Tyson (a huge, homeless crybaby) is Percy’s only friend at his new school. When the school took him in off the streets as a project, Percy befriended him to protect him. During gym class, the school bully, Sloan, has large friends visiting. They start playing dodgeball, but most of Percy’s team runs away. The six visitors grow into monsters and start throwing huge balls of fire. Tyson seems to be able to handle them and kills all but the leader. Percy looks through the ruins of the locker room to find his sword, Riptide, but Annabeth ends up killing the leader from behind. They flee the scene as adults start to blame Percy.
Annabeth is very unfriendly towards Tyson and Percy can’t understand why. She says they better bring him along and calls a special cab. Three old ladies that share an eye and a tooth drive them. One mentions that Percy is seeking a location, but he doesn’t know what they are talking about. He ends up with their eye after they fight over it. He demands they explain. They give him four numbers (30, 31, 75, 12) and he gives it back. They can’t explain anymore and Annabeth points out their fellow campers are already in battle.
The campers are fighting off two large, bronze bulls inside the camp’s boundary. Percy immediately runs to help. While dodging one, he hurts his leg. He can’t move well enough to get out of the way of the second bull. Annabeth invites Tyson into camp and he steps between Percy and the bull. The beast spits fire at Tyson, but he is unharmed and beats the machine to scrap. Clarisse has taken out the second one. Annabeth tells Percy that Tyson is an orphaned Cyclops. Clarisse comes to yell at them for ruining her attack. She tells them to help get the injured campers. As they help, they notice that things have changed because Thalia’s tree was poisoned.
Percy, Annabeth, and Tyson walk through the camp that has a completely different feel. They find Chiron packing his saddle bags. The gods don’t think he is fit to run the camp and that he possibly was the one to poison the tree. He tells Percy not to do anything and takes off. At dinner, they meet Tantalus (cursed so that he cannot eat or drink anything) who already has a grudge against Percy. He warns him not to create trouble before sending him to his lonely table. Tantalus announces that they are bringing back chariot races (which have killed demigods in the past). Clarisse points out that the reward (no chores) could be dangerous if her cabin is not on guard duty. Tantalus is not concerned and moves on to putting Tyson in a cabin. Poseidon claims him.
Not knowing what else to do, Percy and Annabeth start working on their chariot. It isn’t long before they have a fight. Percy learns how to ride a pegasus. He is mad at Poseidon for comparing him to Tyson. At night, Grover talks to him through an empathy link to tell him he is trapped by the cyclops, Polyphemus, who currently thinks Grover is his bride. The satyr tells Percy to go to the Sea of Monsters, but warns him that if he is killed, it could kill Percy as well. Percy tells Annabeth about Grover before the race, but she thinks he is trying to distract her so he can win. He offers to go to the Oracle (which she knows he hates). The race starts and then Stymphalian birds attack. Percy and Annabeth team up to get to the Big House. They grab Chiron’s boom box and head back to the arena. The noise of Chiron’s music drives the birds away and the Apollo cabin has a chance to shoot them down. Tantalus decides to punish Percy, Annabeth, and Tyson for disturbing the race.
Percy, Tyson, and Annabeth are given kitchen cleanup duty as punishment for disturbing the race. percy recounts his dream and Annabeth explains that the Golden Fleece (heals whatever land it is in) is what has been attracting the satyrs that don’t return from their search for Pan. At the campfire that night, Percy tells the story and gives the coordinates the Grey Sisters gave him. Tantalus decides to send Clarisse on the quest. When people start to protest, he tells them his story. He stole some food from the gods in hopes of recreating it. When they gods found out, they were not happy and punished him. He decided to get his revenge by feeding them his children. He warns that curfew will be strictly enforced and anyone that leaves without permission will be expelled. That night, Percy sneaks down to the beach to think. Hermes gives him a thermos containing the winds of the four corners of the earth and a bottle of multivitamins. Percy asks why Hermes is helping him and realizes he wants him to help Luke. Annabeth and Tyson start to approach and the god leaves him with three packed duffle bags and points out a ship in the distance. Percy has to decide quickly.
Annabeth and Tyson come running and they both decide to go with him. He goes into the water and asks for his father’s help. Three hippocampi (horses with fish tails) arrive to take them to the cruise ship, the Princess Andromeda. The ship seems to be deserted, so they look around to find a safe place to sleep. Annabeth warns them not to eat or drink anything. Percy dreams of Kronos mocking him for going on another quest for the gods. He then dreams of Grover weaving in the cave. They all wake up and decide to leave the ship after seeing humans in a trance and monsters mixed together. However, they hear Luke talking about a trap he has set and they decide to find out what that is.
They follow Luke and Tyson can hear him talking to Agrius and Oreius (mom was made to fall in love with a hear and had the twins). Luke discovers them quickly and tries to get them to join his side. Kronos gets stronger with each demigod that joins them. He has a spy at the camp and already knows their plans. He sends Oreius to feed them to the Aethiopian dragon. They are able to get away and escape to a life boat.
Percy uses the thermos to land and make a speedy getaway from the cruise ship. Annabeth leads them to a hideout and percy sends Tyson to look for a wilderness store. Annabeth explains that Percy and Thalia are a lot alike. She indicates that it was a cyclops that killed her. Tyson comes back with donuts and points out a chain store close by. Annabeth explains that the chains are all connected for breeding monsters. A hydra appears, but they don’t have a way to kill it without fire to keep the heads from multiplying. Clarisse comes along on the CSS Birmingham and destroys it with a shot from one of her canons.
Clarisse shows them around the ship and refuses to accept their help (even though her cabin mates turned down the opportunity to join her). They are taken to the sleeping quarters and Percy dreams about Grover. Polyphemus has caught him unraveling the bridal train and gives him magic wool that won’t unravel. He has one day to finish. Percy is woken up because they are about to enter the Sea of Monsters. They have to pass between Charybdis (sucks up and spits out the sea) and Scylla (snatches sailors from the ship). Annabeth suggests an alternate route through the clashing rocks, but Clarisse insists they head towards Charybdis. As the mouth breaths in, they are sucked forward too fast. Tyson goes to fix the ship’s engine. They fire cannons, but when the monster breathes out, they are fired back and crack the haul of the ship. The engines are about to explode, so percy goes after Tyson while the others get to the life boats. They are pushed back too far and Scylla snatches Percy. He is able to get himself free by stabbing her in the eye. As he falls from the monster dropping him, the ship explodes and sends him flying into the water.
Percy wakes up on one of the lifeboats with Annabeth. She used the last of the wind in the thermos to get them away. He asks her about the prophecy Luke mentioned and she tells him that he first child of one of the Big Three to turn 16 would be a weapon to save or destroy the age to the Olympians. They reach an island with a spa and are taken to meet C.C. (Circe). She immediately gets Percy and Annabeth to agree to separate. She shows Percy what he could look like if he drinks her mixture. He does and is turned into a guinea pig. Annabeth returns and realizes something is wrong when Percy is missing. She asks Circe to say goodbye to Percy and the room is locked. She grans the multivitamins and takes one. She is immune to magic temporarily and gets her dagger to the sorceress’ neck when she returns to the room. She gives the guinea pigs vitamins and they all change back. One of them is Blackbeard, a son of Ares. Percy and Annabeth run out and board the Queen Anne’s Revenge. Percy is able to sail it on his own and they leave.
After sailing through the night, Annabeth tells percy that when they first went to camp, she, Luke and Thalia got lost in a cyclops’ maze. Annabeth was able to free Thalia, who took care of the rest, but it gave the other monsters that had been following them time to catch up. Percy dreams about Thalia and her shield in Luke’s conference room. She goes to kill Kronos in his sarcophagus (where he is being reformed), but she is sucked in. Annabeth wakes him up and warns him they are approaching the sirens. She wants to listen to their song because it is rumored to make a person wiser. Percy ties her up and plugs his own ears with wax. he looks away from her because she looks miserable. When he looks back, she is already swimming for the sirens. He catches up and when he grabs her, he sees her dream; New York city completely rebuilt by her stands tall while her parents and Luke invite her to join them. Percy realizes that the song can’t be heard underwater and pulls her down. He forms an air bubble around them to get them back to the ship and out of the Siren’s territory. Annabeth learned that her fatal flaw (all demigods have one) is hubris. They reach the cyclops’ island.
As they make their way towards the cave, Percy and Annabeth find the Golden Fleece surrounded by sheep that eat an entire dear in seconds. They decide to worry about that after they save Grover. They cross the rope bridge and start climbing the cliffs to the cave. Once inside, they see Grove and Clarisse sealed into another room with a boulder. They wait until nightfall when Polyphemus will bring his regular sheep into the cave. Percy attaches himself to the bottom of one to get in. Annabeth distracts the cyclops by telling him she is Nobody. Polyphemus chases her and Percy has a chance to rescue the others. Clarisse tells him that no one else survived on the Birmingham. Annabeth screams.
Polyphemus holds Annabeth so Percy sends Clarisse and Grover to sneak up behind him and attack from either side. The cyclops drops Annabeth and Percy has Grover get her out. They make a run for it and make it over the rope bridge to where the man-eating sheep are. They try to cut the bridge but Polyphemus still has enough time to reach them. Percy is able to take him down and trusts that he will let them leave with the Golden Fleece. As soon as Percy releases him, the cyclops goes back on his word. Tyson arrives and throws a rock down Polyphemus’ throat and knocks him off the cliff. He gets the fleece to Percy and they use it to heal Annabeth. Tyson distracts the sheep so they can get to the ship. When they start going into the water, Polyphemus comes back from over the cliff.
Tyson and Percy fight off Polyphemus while the others head for the ship. They are able to get away and start swimming to join the others. Clarisse starts yelling insults at the cyclops. He is able to sink their ship by throwing a boulder at her voice. Percy and Tyson call on the hippocampi to help their friends.
The hippocampi drop them off the shore of Miami. Percy realizes they don’t have much time to get the fleece to Thalia’s tree. Tyson produces the money Hermes gave them and Percy sends Clarisse to the airport with the fleece. Luke finds them shortly after and takes them back onto the cruise ship. He intends to go after Clarisse, but while he is waiting for his steed, Percy has Luke repeat what he has done and plans to do. Unfortunately for Luke, Percy sent an Iris message to Mr. D, which proves Chiron’s innocence in the situation. Luke calls out more guards to surround them.
Percy challenges Luke to fight one on one. A black pegasus mare is brought out and percy can hear how much she hates Luke. percy and Luke fight, but Percy’s leg is hurt and it looks like he is going to loose. Suddenly, Chiron and his cousins ride in and save the day. They are able to get them away from the ship and to their camp. Chiron doesn’t know if Percy is the one in the prophecy, but he thinks Kronos will kill Percy because he won’t join his side to end Western Civilization.
They get back to camp in time to see Clarisse put the fleece on Thalia’s tree. Everything is healed and even better than before. Chiron decides to keep the chariot races . When Percy is preparing their horses the night before, Hermes comes to him. He apologizes to the god for not being able to get Luke back on their side. Hermes gives him a letter from Poseidon that just says, “Brace yourself.” Thanks to improvements to the Athena chariot, the Athen/Poseidon team win the race. Percy publicly thanks his little brother for his help.
Percy calls his mom to let her know he is safe. Chiron sorts out the legal matters with his last school using the mist. Tyson leaves camp to work in the Cyclops forges. Grover (stayed at camp on furlough) wakes Percy up one morning because something happened while Annabeth was guarding the tree. The fleece healed Thalia back into living human form.