In an alternate reality of 21st century Japan, Tokugawa Shogunate has remained in power and is isolated from the rest of the world. Muneakira Yagyu travels to Buou Academy, where he is to become the master of its Dojo. In this academy, students are trained to become samurai warriors. A girl suddenly falls from the sky into his arms, calls him “big brother” and kisses him, which causes her to transform into a master samurai.