Percy, expelled from six schools for being unable to control his temper, learns the truth from his mother that his father is the Greek god Poseidon, and is sent to Camp Half Blood where he is befriended by a satyr and the demigod daughter of Athena who join him in a journey to the Underworld to retrieve Zeus's lightning bolt and prevent a catastrophic war.

Seventh-grader Percy Jackson has recently discovered that he is the son of Poseidon, making him half human and half god, and along with a demigod and a satyr friend, must save his beloved Camp Half-Blood from evil forces determined to destroy it.