Although mathematically gifted, Junki Saiga is otherwise a typical high school student, whose archaeologist father disappeared during an expedition years ago. But recently his family learned that his father's remains have been discovered near Japan's pyramid, along with artifacts and notes from his research. While Junki is still reeling from the confirmation of his father's death, a meteor comes crashing from the sky carrying a foreign robotic life-form that immediately begins attacking anything and everything. And this sequence of strange events triggers a bracelet left from his father's research to activate and merge with an ancient robot hidden inside the pyramid: Reideen. But not everyone welcomes Reideen's help in dealing with the alien threat. There are those that want the robot's power for themselves, and still others who would see it and its pilot destroyed. Without the help of his nation, or even his friends, Junki must push himself to succeed in piloting Reideen against the aliens or watch the world perish!