In third century mythical Japan, a young prince named Izumo lives in the land of Nakatsukuni. Unlike his diligent cousin Dekiru, Izumo is restless and longs to see the wider world beyond his kingdom. His opportunity for adventure arrives when a mysterious girl named Sanae is discovered as a stowaway on an airship. Though she claims to be from the kingdom of Yamataikoku, Sanae is secretly a member of the Naga, a shadowy people who possess magical powers.

Her true identity places her in grave danger. Sanae is kidnapped by Takeru, a fearsome warrior from the rival kingdom of Akusa, who delivers her to the evil witch Yomihime. With Sanae held captive, Izumo refuses to stand idle. Accompanied by Navi, an orphaned glider pilot, he sets out on a rescue mission. To succeed, Izumo must first take up the sacred sword of his country and learn to master its legendary powers. His journey leads him to a confrontation with a reawakened mythical beast, the eight-headed serpent Yamata-no-Orochi, a challenge he must overcome before he can hope to save Sanae from Yomihime's clutches.