徳川11代将軍・家斎の世。伴天連・ジュリアン・ヘルナンドが幕府大目付・松平主水正の娘を黒ミサの儀式において犯すという事件が発生。その結果として生まれた子・眠狂四郎が、名刀無想正宗を腰に市井をぶらつき遭遇する事件と美女…。ニヒルな魅力で田村正和が人気を得た作品で原作者の柴田錬三郎氏から太鼓判を押されたという。
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.