The wandering assassin Ogami Itto and his young son, Daigoro, continue their journey along the demon path in feudal Japan. Itto, once the Shogun’s executioner, now walks the road of vengeance against the Yagyu clan who destroyed his family, hiring out his sword and the contents of his son's baby cart to fund his quest.

On the road, Itto is confronted by a series of five veiled warriors, each wearing a mask depicting the beasts of hell. These are messengers from the Kuroda clan, and each challenges him to a duel. As Itto defeats them one by one, each fallen warrior passes on a portion of the five hundred ryo assassination fee and reveals a fragment of the mission's secrets. The puzzle pieces describe a deadly conspiracy within the Kuroda house: the aging lord has no legitimate male heir. He has imprisoned his true son and raised his daughter, born of a concubine, as a boy to be his successor. A document detailing this deception has been entrusted to a high priest named Jikei, who is secretly a ninja agent. If the priest delivers the letter to the shogun, the Kuroda clan will be dissolved, leaving its samurai and vassals ruined. Itto is hired to intercept and kill the priest and retrieve the letter before it reaches their mortal enemy, Yagyu Retsudo.

While passing through a town during a festival, Daigoro is separated from his father. He encounters a female pickpocket named Oyo, who is fleeing the authorities. She hands him a stolen wallet and extracts a promise of silence. When the constables capture Daigoro, they beat him publicly to force him to reveal the thief's identity, but the child refuses to break his word. Even when Oyo reveals herself and confesses, Daigoro’s stoic endurance impresses the officials, and he is released, having proven his honor.

Continuing the mission, Itto is also contracted by a young woman named Shiranui to kill the lord, his concubine, and the daughter raised as a boy. To complete the primary task, Itto performs a feat of martial prowess, swimming underwater to a boat to steal the incriminating letter from the high priest. After securing the document, he storms the lord’s compound. He battles through the lord’s elite guards, ultimately killing the lord, the concubine, and the disguised girl, thereby fulfilling both contracts.

Having prevented the political disaster, Itto prepares to depart by boat. The woman Shiranui appears on the shore. She reveals that the true heir—the imprisoned son—will now rule, and the official story will be that the retired lord committed seppuku to preserve the clan's honor. With her duty fulfilled, she commits harakiri before the lone wolf and his cub as they sail away on the river.