舞台是近未来的日本，过着普通高中生生活的才贺淳贵听闻10年前便下落不明的父亲的遗体在岩手县黑神山的遗迹中被发现，便急忙赶赴那里。然而才贺在遗迹中却看到了不可思议的幻觉。他的手腕上出现了一个金色的腕轮，上面刻着神代文字“使用者 REIDEEN”。与此同时，巨大的陨石从空中落下、湖中出现了巨大的机器人……Saiga is a normal high school student with a gift in mathematics. His daily routine is disrupted when his family gets news that his Father's remains have been discovered-a noted archeologist and researcher who had gone missing while exploring a site many years before. Among his 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 and merge him with an ancient robot burried within the pyramid-a robot the runes describe as Reideen. It is now up to Saiga and guardian Reideen to fight against this unknown alien threat from the sky.
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