After a 12,000‑year slumber, the Demon Empire returns to seize control of Earth. The giant, robot‑like guardian of the lost continent of Mu, Reideen, detects the evil presence and awakens from its golden pyramid. It reveals to young Japanese boy Akira Hibiki that he is the descendant of the ancient Mu people and the one who must help Reideen save the world. Akira is aided by his friends, including the token girl Mari Sakurano, daughter of a scientist fighting the Demon Empire, and several members of his high‑school soccer team.