We start off with Percy now thirteen years old having nightmares of his best friend Grover. The first involving him running from an unseen
force and taking refuge in a wedding dress shop. The next day when Percy goes to school he gets attacked by of Laestrygonians and his
classmate Tyson helps save him. Annabeth meets him and takes Percy and Tyson to Camp Half-Blood in a taxi by the Gray Sisters. Annabeth then
reveales to Percy that Tyson is a young Cyclops. Once they get to the camp it is under attack by two Bronze Colchis bulls which Tyson
defeats. That's when everyone learns that Thalia's tree has been poisoned leaving the camp bulnerable to enemy attacks and that Chiron is
suspected of poisoning it due to his family connections to Kronos.
After the battle Percy tells Annabeth of his dreams when she decided that Grover has found the Golden Fleeze which could cure Thalia's tree. After all the campers are attacked at a chariot race by Stymplalian bird, Tantalus (who is replacing Chrion), sends Clarisse, daughter of Aries, on a quest to find the Golden Fleeze. Though they weren't chosen Percy, Annabeth and Tyson run away from camp in order to find Grover Hermes, god of messengersm gives them gifts to help them on their quest and tells them to call on some Hippocampi, who carry them to a cruise ship, the Princess Andromeda. After they get on board the three of them learn that it looks deserted but find everything fully stocked and go to sleep. In the morning the wake up to find it populated with monsters. They are taken by Luke Castellan, who is taking an army of monsters to a different land.
Luke sends them to be fed to a drakon but they escape on a lifeboat and find land on a swamp area. There they find out a hideout that Luke, Annabeth and Thalia built when they were children. Annabeth explains everything about Luke while Tyson looks for doughnuts to eat. They go to the doughnut shop, Monster Donut, but a hydra appears and tries to kill them when they are saved by Clarisse who is sailing on a confederate ship called the CSS Birmingham.
On their way to the Sea of Monsters, the ship is destroyed by the monster Charybdis and then attacked again by Scylla. Thinking Tyson and Clarisse are dead, Percy and Annabeth escape to a nearby island where they find the "CC's Spa and Resort." CC has an assistant that gives Annabeth a tour telling her that Percy needs a makeover. She shows Percy the best versions of himself using magic and gives him a magic drink. When he drinks it he then turns into a guinea pig and is put into a cage ith six other mean looking ones. When Annabeth comes back CC reveals herself as the sorceress Circe who turns men into guinea pigs. Annabeth asks for one last moment with Percy and then uses Hermes anti magic vitamins to protect herself from Circe's magic and turns Percy and the others back to normal. She learns that the other men used to be the crew of Blackbeard (the son of Ares). They all manage to escape Circe's island and steals Blackbeard's ship. The Queen Anne's Revenge.
Percy and Annabeth reach Polyphemus' island, where they are reunited with Tyson and Clarisse who somehow survived the shipwreck. They rescue Grover and find the Golden Fleece and are returned to the mainland by Hippocampi. Clarisse returns to Camp Half-Blood with the Fleece by plane. Percy, Annabeth, Grover and Tyson are taken by Luke and brought onto the Princess Andromeda. Percy somehow gets in contact with Camp Half-Blood through Iris Message and tricks Luke into admitting that he poisoned Thalia's tree, proving Chrion is innocence in front of the entire camp. Luke duels Percy, beats him and is about to kill them but before he can Chiron and the party ponies rescure them. Returning to camp, Chiron is given his job back and the Golden Fleece heals Thalia's tree. Hoever it also makes Thalia Grace come out the tree.