Zankuro Matsudaira (Ken Watanabe) is a low-grade vassal of the Tokugawa Shogun, forced to moonlight as a bodyguard and executioner to make ends meet. He also has a drinking problem, an overeating mother, and a propensity to protect the oppressed, all of which eat into his finances. His confident mother Masajo (Kyoko Kishida) is the descendant of a famous family of shogunate retainers and a mistress of the pen and sword, who regards her son as a failure.

名門の家柄だが、無役で微禄・最下級の御家人である松平残九郎"通称：斬九郎"（渡辺謙）は、用心棒から首斬りまで数々の裏稼業を請け負いながら、グルメな母親・麻佐女（岸田今日子）を養う身であった。母には頭が上がらず、気っ風の良い深川芸者の蔦吉（若村麻由美）とは相思相愛の間柄ながらも今一歩進展しない斬九郎だが、弱い者や正しい者には味方となり、豪快な太刀さばきで悪を討つ！