Steam became the cornerstone of civilization in the Edo period, giving rise to the creation of massive machines called the onigami. The prestige of the Tokugawa Shogunate was unshakable, and there wasn't even the slightest hint of rebellion. A bathhouse worker, Keiichirou is shocked to discover that he is the secret heir to the dying Tokugawa shogun. While continuing to work in the bathhouse, he has to master the legendary onigami, Susanoo. The mecha can only be piloted by a "perfect virgin of the Tokugawa blood" and a female virgin co-pilot, thus the choice lies between a female ninja (Hattori Kiriko) and a fox-girl (Chiharu).