Saiga is an ordinary high school student with a talent for mathematics. His daily routine is disrupted when his family receives news that his father's remains have been discovered. His father was a noted archaeologist and researcher who had gone missing while exploring a site many years prior. Among the remains were notes and artifacts that needed to be identified by the family near a notable triangular mountain in Japan known as "Japan's pyramid", a place suspected by some to be man-made. A meteor containing a strange robotic lifeform falls from the sky and begins to cause destruction, putting Saiga in danger and causing a mysterious bracelet from his father's research to activate. This merges him with an ancient robot buried within the pyramid—a robot the runes describe as Reideen. It is now up to Saiga and his guardian Reideen to fight against this unknown alien threat from the sky.