The novel introduces the protagonist, Percy Jackson, who is a teen demigod. He is the son of a mortal woman and son of Poseidon, god of the sea. Percy spends his summers with other demigods at Camp Half-Blood, which is located in Long Island, NY.

One afternoon in August, he is driving with his friend and love interest Rachel, a mortal. A pegasus named Blackfoot lands on the hood of the car. The animal is carrying Charles Beckendorf, an 18-year-old counselor at the camp. Percy and Charles are in the midst of a war against Kronos, lord of the Titans. His mission is to destroy Mount Olympus, now located in Manhattan, and Percy and Charles fight against him. Kronos has taken over the body of Luke, a former member of Camp Half-Blood and Percy’s nemesis.

Percy joins Charles on Blackfoot to fly to the Princess Andromeda, a cruise ship that is Kronos’s headquarters. The two plan to detonate the ship, and they go to the engine room. They encounter telkhines (a half-dog, half-man being), and Percy uses his sword, Riptide, to attack them, turning them into dust.