After returning home for the school year, Percy Jackson finds himself at Meriwether College Prep, another new school, with a hulking new classmate named Tyson, who, despite his six-foot-three frame, is a big softie. Percy and Tyson are each other’s only friends. Meriwether is a “progressive” school, which simply means, according to Percy, there are beanbag chairs instead of desks and no grades.
The school year seems to be going fine. Seems to be, anyway, until uninvited guests show up for gym class and start firing flaming bronze cannonballs. That’s the sort of thing that happens when you are the son of the sea god - half mortal, half god, and not really part of either world.
After escaping and meeting up with Annabeth, she gives Percy and Tyson the details about events at Camp Half-Blood. Thalia’s tree, part of the magical border that protects Camp Half-Blood, has been poisoned. The tree is dying and along with it, the camp’s magical protection. Percy, Annabeth, and Tyson set off on a quest for the Golden Fleece. The Fleece’s magic powers of healing may just bring health back to Thalia’s tree.
Meanwhile, Percy’s satyr friend, Grover, has set off as a searcher hoping to be the first satyr to find Pan. His search has led him into danger, and only a wedding dress, falsetto voice, and his enemy’s horrible vision and sub-par mental capacity have kept him alive. An empathy link between Grover and Percy is formed, allowing them to communicate over great distances, but it also endangers the other. If one of them is killed, the other will die as well.
Percy’s quest brings him, Annabeth, and Tyson closer to both the Golden Fleece and imprisoned Grover. The quest also takes them directly through the Sea of Monsters. Everything should be against them – Percy’s father and Annabeth’s mother, Poseidon and Athena, dislike one another and Tyson is a Cyclops, and nobody likes Cyclops. But in the end, friendships, both old and new, remain strong, and Tyson proves himself a loyal friend (and more).