The series is about the adventures of modern 12-year-old Percy Jackson after he discovers he is a demigod ( half-human , half-god ) . Percy is the son of a mortal woman and the Greek god Poseidon . It is the first novel in the Percy Jackson & the Olympians series , which charts the adventures of a modern-day twelve-year-old Percy Jackson as he discovers he is a demigod , the son of a mortal woman and the Greek god Poseidon .