When Percy's mom finds out, she knows it's time that he knew the truth about where he came from, and that he go to the one place he'll be safe.
She sends Percy to Camp Half Blood, a summer camp for demigods (on Long Island),where he learns that the father he never knew is Poseidon, God of the Sea. Soon a mystery unfolds and together with his friends Percy sets out on a quest across the United States to reach the gates of the Underworld (located in a recording studio in Hollywood) and prevent a catastrophic war between the gods.