After a slumber of twelve millennia, the Demon Empire awakens 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. A young Japanese boy, Akira Hibiki, is alerted about the Demon Empire by a mysterious voice and rushes to the pyramid.
It is soon revealed that Akira is a descendant of the ancient people of Mu who must help Raideen save the Earth. Akira enters the robot by accelerating his motorcycle to a high speed and then throws himself upward, allowing a beam from the robot to pull him into the robot's head and into an internal cockpit from where he assumes control of Reideen. Akira is aided by Mari Sakurano, who happens to be the daughter of a prominent scientist, and his friends from the soccer club. Halfway through the story, the Demon Empire's master, Barao, is released from his statue prison and intends to finish what he started twelve thousand years earlier.
