A dead father's mystery pulls you into something that was always watching you. The first ten episodes move like archaeology, patient, methodical, building layers you won't understand until they collapse. Once it stops pretending to be a high school show, Reideen becomes about a kid learning that the entire universe has been waiting for him to exist, and the animation shifts to match that weight, 3D and 2D moving together like something ancient waking up. This is mecha that earns its slowness.
Saiga lives an ordinary life as a high school math prodigy until his father, a missing archaeologist, is finally found dead near Japan's most enigmatic landmark: a massive triangular mountain rumored to be an ancient pyramid. The discovery sets off a chain of impossible events. A meteor crashes to Earth, unleashing a mechanical entity that ravages everything in its path. In desperation, an artifact Saiga inherited from his father activates, yanking him into the cockpit of Reideen, a colossal robot entombed beneath the pyramid for millennia. Now Saiga must pilot this ancient war machine to defend humanity from the encroaching threat, but every battle raises darker questions: why was Reideen buried here? What does the meteor's arrival really mean? As he masters his newfound power, Saiga uncovers the truth about his father's research and realizes this isn't humanity's first encounter with catastrophic forces from the sky. The line between savior and weapon grows dangerously thin.