When Thalia's tree is mysteriously poisoned, the magical borders of Camp Half-Blood begin to fail. Now Percy and his friends have just days to find the only magical item powerful enough to save the camp before monsters overrun it. The catch: they must sail into the Sea of Monsters to find it. Along the way, Percy must stage a daring rescue operation to save his old friend Grover. He also learns a terrible secret about his own family, which makes him question whether being the son of Poseidon is an honor or a curse.