Graphically adapts the adventures of the Greek god of the seas, presenting the myth of how Poseidon became the king of the oceans and such stories as Theseus and the Minotaur, Odysseus and Polyphemos, and the founding of Athens.

After learning that he is the son of a mortal woman and the god of the sea, twelve-year-old Percy is sent to a summer camp for demigods like himself, and joins his new friends on a quest to prevent a war between the gods.