After a summer spent trying to prevent a catastrophic war among the Greek gods, Percy Jackson finds his seventh-grade school year unnervingly quiet. But things don't stay quiet for long. Percy soon discovers that the magical borders which protect Half-Blood Hill have been poisoned by a mysterious enemy, and the only safe haven for demigods is on the verge of being overrun by mythological monsters. To save the camp, Percy needs the help of his best friend, Grover, who has been taken prisoner on an island somewhere in the Sea of Monsters, the dangerous waters Greek heroes have sailed for millennia.