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