Set in the eighteenth century, during the reign of the eleventh Tokugawa shōgun Tokugawa Ienari. Nemuri Kyōshirō is a master of the sword, but he is a man who shuts his mind and heart because of his unhappy background: Kyōshirō's father was a Dutch missionary who became an apostate and his mother was the daughter of Matsudaira Mondonoshoū, the Tokugawa shogunate's upper superintendent officer. He was an unwanted child conceived during a Black Mass ceremony, and Kyōshirō's mother killed herself soon after Kyōshirō`s birth. Although he hates being involved with incidents and people, he is always involved in them no matter where he goes. Some people challenge him to a duel to see his unusual Engetsu Sappō (Full Moon cut) sword style, while others try to use him because he is a half-breed. He solves incidents with both his intelligence and his Musō Masamune sword.