Ep. 1: Junki Saiga discovers a golden bracelet and a giant robot after his father's remains are found.

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Junki Saiga, a high school student living with his mother and sister Kuraka, believes giant robot stories are fictional. In the near future, police inform him that the remains of his father, an archaeologist who disappeared ten years ago, were discovered at Kurokami Mountain in Iwate Prefecture. The family travels to the site, where Junki loses consciousness and experiences a strange vision. Upon waking, he holds a golden bracelet inscribed with "The Great Reideen." That night, a meteorite strikes Earth, and a massive mechanical monster emerges from a nearby dam's artificial lake. Simultaneously, a gigantic golden figure appears on Kurokami Mountain, suggesting Junki has acquired a titanic power that may bring glory or terror.