Peace in 17th-century Japan causes the Shogunate's breakup of warrior clans, throwing thousands of samurai out of work and into poverty. An honorable end to such fate under the samurai code is ritual suicide, or hara-kiri (self-inflicted disembowelment). An elder warrior, Hanshiro Tsugumo (Tatsuya Nakadai) seeks admittance to the house of a feudal lord to commit the act. There, he learns of the fate of his son-in-law, a young samurai who sought work at the house but was instead barbarically forced to commit traditional hara-kiri in an excruciating manner with a dull bamboo blade. In flashbacks the samurai tells the tragic story of his son-in-law, and how he was forced to sell his real sword to support his sick wife and child. Tsugumo thus sets in motion a tense showdown of revenge against the house.

After being forcefully inducted as a soldier into war in 14th century Japan, his wife and mother remain living in a swamp. They eke out their living by ambushing worn-out warriors, killing them and selling their belongings to a greedy merchant. The woman comes to mistrust her daughter-in-law who has coupled up with a deserter, and begins to wear a facial mask she has taken from a slain samurai. Soon the mask will not come off again. In this disguise she is at first taken for a demon by her daughter.