The herds of magical horses in Horsa have a fragile alliance, but when a prophecy stating that a yearling with a new kind of magic has been born, the four herds each elect a group of horses to find the yearling and bring balance to Horsa.

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.