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.

Carter Kane and his sister must prevent the chaos snake Apophis from breaking free in a few days' time or the world will come to an end. To have any chance of battling these Forces of Chaos, the Kanes must revive the sun god Ra.