Percy's seventh year at school is going rather quietly, if it weren't for those nightmares in which his best friend is in danger. And indeed: Grover is in the power of a cyclops. To make matters worse, Camp Half-Blood is no longer safe: Someone has poisoned the Tree of Thalia, which has protected the boundaries of this magical place until now. Only the Golden Fleece can help there. Will Percy be able to save Grover and the camp? That shouldn't really be a problem for a demigod like him. But his enemies still have some nasty surprises in store.