After learning that the father he never knew is Poseidon, God of the Sea, Percy Jackson is transferred from boarding school to Camp Half-Blood, a summer camp for demigods, and becomes involved in a quest to prevent a catastrophic war between the gods.

When the goddess Artemis disappears while hunting a rare, ancient monster, a group of her followers joins Percy and his friends in an attempt to find and rescue her before the winter solstice, when her influence is needed to sway the Olympian Council regarding the war with the Titans.

The long-awaited prophecy surrounding Percy Jackson's sixteenth birthday unfolds as he leads an army of young demigods to stop Kronos in his advance on New York City, while the Olympians struggle to contain the rampaging monster, Typhon.