In ages long past, the peaceful realm of Mu is under dire threat from the expansionist Dragonian Empire. With scarce military resources and only a giant god statue for protection, Queen Aira and her people are helpless against the Dorado, the Golden Monarch, and his army of dinosaurs.
Yet Aira pins her hopes on a champion from a different time. She reaches out to Yamato Hino, a seemingly ordinary but highly athletic high schooler living in modern Tokyo. Initially, Yamato dismisses the spectral messages from the youthful queen as hallucinations, but she eventually persuades him to cross through a magical portal into ancient Mu.
Soon after Yamato's arrival, Dragonia launches another offensive, but this time the invaders are driven back when the enormous God Mazinger merges with Yamato. Though Mu has achieved a first victory, much work remains. Yamato must also undertake a personal quest, confronting his own flaws and growing into a true warrior to safeguard the kingdom and discover the path back home.