The story is set in the Edo period of feudal Japan. It revolves around Yamada Asaemon, nicknamed Kubikiri Asa (literally "Neck-chopper Asa", often transliterated as "Decapitator Asaemon"), a ronin who is responsible for testing new swords for the shogun. The character is based on a real-life line of sword-testers who served the Tokugawa Shogunate up to the early 19th century.
-- Wikipedia
