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.

Zeus is betrayed by Hades, captured and brought to the Underworld, and Perseus must abandon his attempts to retire to a quiet life as a fisherman, find Zeus, and end the Titans' struggle to rule the Earth.

When Camp Half-Blood, the only safe haven for demigods, comes close to being overrun by mythological monsters, Percy must find his best friend Grover, who is prisoner on an island in the Bermuda Triangle, and then set out to save the Camp.

The myth has been told for centuries: Brothers Zeus, Poseidon, and Hades waged a years-long war against their father, Cronus, and the tyrannical Titans. Upon their mighty victory, Zeus split the world into three. For himself, he claimed the Sky and what fell below it. To his brother Poseidon, he assigned dominion over the Sea. And his oldest brother, Hades, would rule the Underworld. While Hades tried to make the best of a lonely situation, eternity...