During school, Percy is stuck playing dodgeball against large seventh graders who turn out to be Laistrygonians. They summon explosive fireballs and attempt to kill Percy, but he is protected by from behind and takes Percy and Tyson back to Camp Half-Blood. They find the camp under attack because Thalia's tree, which protects the camp, has been poisoned by an unknown intruder. Chiron, the activities director, has been fired because he is suspected of poisoning the tree, and is replaced by Tantalus, a guy from the Fields of Punishment.

          Percy finds out that Tyson is a Cyclops and Poseidon claims him as his son. Annabeth and Percy ask Tantalus to send somebody on a quest to find the Golden Fleece, which he does, sending Clarisse. Percy receives help from Poseidon, who gives them three Hippocampi, and together with Annabeth and Tyson, ends up on a passing cruise ship, the Princess Andromeda, which is revealed to be owned by Luke. They are captured and learn that Luke is trying to reform Kronos, a Titan who is the father of Zeus. They manage to escape and are later saved by Clarisse, who has a boat of her own that was given to her by her father Ares. It is destroyed upon entering the Sea of Monsters, known to mortals as the Bermuda Triangle and Tyson is presumed dead after the ship exploded. Along the way, they pass the land of the Sirens, and Annabeth, who knows that the Sirens tell one of their innermost desires, decides that she wants Percy to tie her to mast, and have her listen to the Sirens' songs. However, Percy forgets to remove her knife, and she manages to free herself, and almost reaches the land, but Percy manages to save her; in doing so, he learns that the Sirens' song made Annabeth see what she wanted most: her parents reunited and Luke converted back to good, all having a picnic, in front a brand new Manhattan, rebuilt by Annabeth. They reach the island of Polyphemus-where they find Tyson safe and alive-and save Grover with the help of Tyson and Clarisse, recovering the Fleece in the process. They make their way to Florida, and Percy sends Clarisse, with the fleece, back to camp. Percy, Annabeth, Grover and Tyson are captured by Luke. 
         
       Percy manages to contact camp with an Iris-message, tricking Luke into admitting he poisoned Thalia's tree. In a duel with Luke, Percy is nearly killed. He is saved by Chiron and his relatives, the Party Ponies. Chiron is rehired after being proven not guilty, and the Fleece cures Thalia's tree of its poison, and Thalia herself is spewed out of the tree.