For the first time ever, it seems as if everything is going Percy Jackson's way. He's on his last day of 7th-grade and he hasn't been expelled yet. No monsters are after him and the day after he's going back to his favorite place in the world,
Camp Half-Blood. Things of course aren't always this easy for Percy. Suddenly, he's battling a gang of cannibal giants with new friend Tyson in their school gym. Just when Percy thinks they're done for, fellow Half-Blood Annabeth appears and saves them. Percy doesn't even get to go home or say goodbye to his mom before he's off to Camp Half-Blood again.
Immediately upon their arrival, they find some of the campers led by Percy's tormentor, Clarisse fighting monsters on the borders of the camp. Percy and Annabeth manage to save Clarisse and the others with Tyson's help. The barrier which protects the campers have been compromised. The pine tree, which Thalia was turned into after she sacrificed herself to save her fellow Half-Bloods is dying. And this means, that Camp Half-Blood is no longer the safe haven it was. More troubles follow as Percy's been having dreams of his best friend, Grover in danger and now he learns his new friend Tyson, is actually a
Cyclops. And that means that Percy has a monster for a half-brother. Whatever allies Percy have seems to be abandoning or turning against him. Chiron and Argus have been fired for failing to protect Camp Half-Blood and most of the other campers have turned against Percy because of Tyson. Even Annabeth seems to hold a deep prejudice against Percy's Cyclops half-brother.
When Percy and Annabeth make peace and request for a quest to save Camp Half-Blood, the task is given to Clarisse instead. Thus, Percy and his friends find themselves in danger of being expelled from Camp Half-Blood when they decide to go on the quest without permission, with a little goading and assistance from
Hermes. They meet and fight Luke again as well as a host of other monsters on their way to the
Sea of Monsters before finally retrieving the
Golden Fleece and rescuing Grover. Unexpectedly, when they arrive back in the mortal world, Percy lets Clarisse return to Camp Half-Blood with the Golden Fleece on her own.
Percy's generosity proves to be a boon. When Luke catches up with them, he doesn't realize that Clarisse has left with the Golden Fleece at first and thus is delayed by Percy and his friends. Percy uses this opportunity to clear Chiron's name and is rewarded when
Chiron leads his centaur relatives to save Percy's group from Luke's monsters.
Arriving back at Camp Half-Blood, everything seems to return back to normal. The protection of the border is restored and his animosity with Clarisse remains. That is until everyone realizes
Kronos's real intention. He had intended all along for Percy to have the Golden Fleece because with this Thalia is restored to her human form and Kronos has another chess piece in his game against the Greek Gods.