Disturbing dreams and deadly dodgeball—Percy Jackson, son of Poseidon, is having the worst day ever.

First he has a nightmare that his best friend, Grover Underwood, is running for his life. The dream ends with Grover begging Percy to find him before it’s too late. Later at school, a group of muscle-bound bullies gangs up on him during a game of dodgeball. Something about them makes Percy uneasy, and not just because they’re hurling projectiles at him. Then one of them calls him Perseus—his full name, the name of an ancient Greek hero. If they know that name, they know his real identity. And that means. . .

Yep. They’re monsters. Laistrygonian giants, to be exact. They grow before his eyes. Their dodgeballs turn into explosive cannonballs. They take aim. One hit, and he’s a goner.

Help comes from a very unexpected source: Tyson, an oversized homeless kid with a limited vocabulary who Percy has reluctantly befriended. Tyson swats the cannonballs back at the giants, disintegrating them. Percy destroys others by setting off an explosion in the boys’ room. The last giant standing is stabbed by Annabeth Chase, who infiltrated the school by wearing her cap of invisibility—a gift from her mother, the goddess Athena.

Percy, Annabeth, and Tyson take a wild taxi ride to Camp Half-Blood. There, they discover that Half-Blood Hill is under attack by two bronze Colchis bulls. Normally, monsters can’t get through the magical border protecting the camp. But the border has been compromised by an unknown assailant who poisoned the heart of the boundary, a tree known as Thalia’s pine. Soon, the whole border will fail. And when it does, the camp will be overrun with monsters.

Percy’s troubles mount as the days go by. He discovers that Tyson isn’t an overgrown kid, but a young Cyclops and—gulp!—the son of Poseidon, which makes him Percy’s half-brother. He’s nearly killed by demon birds during a chariot race. And Grover is reaching out to him via an empathic link with disturbing news: he’s been captured by Polyphemus, a murderous Cyclops. Polyphemus would likely have eaten the satyr by now, but Grover pulled the wool over his eye by pretending to be a girl keen on marrying him. Unfortunately, Polyphemus is getting suspicious . . . and hungry.

The one bright spot in Percy’s world? He knows how to cure the poisoned pine tree. Draping the branches with the mythical Golden Fleece, an ancient sheepskin with magical healing powers, will do the trick. The Fleece is located on an island in the Sea of Monsters—the same island where Grover is being held captive. All Percy needs is permission to go on a quest, and he can solve two problems at once.

But when the quest is granted, his archenemy Clarisse La Rue, daughter of Ares, is chosen. That doesn’t stop Percy from going on his own unauthorized journey, however, and he takes Annabeth and Tyson with him. Together they head toward the dangerous waters in the Sea of Monsters. But first they visit a ship called the Princess Andromeda, where they make a disturbing discovery—a golden sarcophagus containing pieces of the Titan Kronos, the ancient lord of time. Luke Castellan has pulled them from the bowels of Tartarus where they were cast by the gods. Kronos is re-forming, and when he is whole again, he will take his revenge on the Olympians . . . and the world.