After a 12,000-year slumber, the Demon Empire returns to seize control of Earth. Reideen, the giant robot-like guardian of the lost continent of Mu, senses the evil presence and awakens within its golden pyramid. It reveals to the young Japanese boy Akira Hibiki that he is the sole descendant of the ancient Mu people who must help Reideen save Earth. He is assisted by his friends: the token girl Mari Sakurano (daughter of a scientist fighting the Demon Empire) and several members of his high school soccer team.