In the mid eighteenth century during the Tanuma period in Edo, a retired master swordsman named Kohei Akiyama lives a deceptively quiet life in the district of Kanegafuchi. Though he has set aside his official duties, the fifty nine year old founder of the Mugai ryu style has not lost his superhuman skill with a blade. Kohei shares his home with Oharu, his young wife who is forty years his junior, and navigates his days with a blend of worldly wisdom and a sharp eye for injustice.

His son, Daijiro Akiyama, is a stark contrast to the easygoing Kohei. A serious and straightforward young man of twenty four, Daijiro runs a small, struggling dojo while adhering to the strict austerity of the sword. The two are often joined by Sasaki Mifuyu, a skilled swordswoman who defies the era’s gender conventions by dressing as a man and wielding a tanto dagger with lethal precision. The trio is also aided by Yashichi, a former student of Kohei who now works as a police assistant or informant. The series unfolds in an episodic fashion without a single overarching plot. Each story typically presents a self contained mystery or conflict involving the residents of Edo, ranging from assassinations and stolen swords to dojo rivalries and political corruption. Unlike a standard quest for glory, the narratives focus on the complexities of living as a warrior in a peaceful era, exploring themes of vengeance, economic survival, and the psychological weight of violence. The tone balances hard boiled detective work with poignant human drama, often featuring detailed depictions of the era's cuisine.

Several notable narrative arcs define the relationships and struggles of the characters. The introductory arc titled The Swordswoman binds the trio together when Kohei rescues Mifuyu from an ambush, revealing her defiance of a forced political marriage and prompting the team to investigate a disgraced samurai. In The Four Rulers of Izeki Dojo, Kohei is asked to mediate a succession dispute at the dojo where Mifuyu trains, forcing him to navigate the politics of loyalty versus merit. A later episode, Demon Bear Tavern, showcases the series' focus on redemption as Kohei befriends a gruff, elderly tavern owner known as Demon Bear who hides a secret illness and a past crime involving the parents of the girl he raised as his own. As the series progresses toward the final volume, The Vicissitudes of Life, the political landscape shifts dramatically with the fall of the powerful councilor Okitsugu Tanuma, forcing Kohei to confront his own aging and the end of an era, ultimately resolving the fates of the recurring supporting cast.