Saiga, a regular high school student with a talent in mathematics, faces a life-altering change when his family receives news about the discovery of his father's remains. His father, a respected archaeologist who vanished during an expedition years ago, left behind cryptic notes and artifacts near a prominent triangular mountain in Japan, often dubbed as 'Japan's pyramid'. The area, rumored to be of artificial origin, hides secrets waiting to be unraveled. A meteor carrying a peculiar robotic entity crashes to earth, wreaking havoc and putting Saiga in peril. In a twist of fate, a mysterious bracelet from his father's investigations activates, merging Saiga with an ancient robot entombed within the pyramid—the legendary Reideen. Now bound together, Saiga and Reideen must combat an enigmatic alien menace descending from the heavens.