A disgraced former executioner now working as a ronin for hire, Ogami Itto pushes his infant son Daigoro in a wooden baby cart laden with hidden weapons across feudal Japan under the Tokugawa shogunate. The pair are perpetually hunted by the Yagyu clan, the rivals who framed Itto and murdered his wife. In this fourth installment of the series, Itto accepts a contract from the Owari clan to track down and kill a mysterious assassin who has been slaughtering groups of samurai and cutting off their topknots, a deep dishonor to the dead and their families.

The killer is revealed to be a young woman named Oyuki, a former member of Lord Owari’s personal guard of female swordswomen. After her instructor and trainer, Kozuka Enki, used a secret sword trick to defeat and then rape her, Oyuki fled and marked her body with elaborate tattoos as a symbol of her disgrace and her purpose. On her chest is a kintaro, a demonic child, and on her back a mountain witch. Now she fights bare-breasted, a silent and ferocious warrior who will not stop killing until she has cut down the man responsible for her shame.

While his father is investigating the killings, three-year-old Daigoro wanders away from the baby cart after following a pair of street performers. Lost and alone in the countryside, he endures for days, searching temples and even surviving a fire in a field by burying himself in the mud. His solitude attracts the attention of Gunbei Yagyu, the disgraced son of the Yagyu clan lord Retsudo. Years earlier, Gunbei and Itto competed for the position of the Shogun’s executioner; Gunbei lost the post when his over-zealous sword style caused him to point his blade at the Shogun himself. Seeing the fearless defiance in Daigoro’s eyes, Gunbei recognizes the child as the son of his old rival and prepares to kill him, but Itto arrives just in time. The two ronin engage in a rematch, and Itto swiftly severs Gunbei’s sword arm before refusing to deliver the killing blow, declaring there is nothing to be gained from killing a man who is already dead.

Reunited with his son, Itto continues his hunt. He learns that Oyuki once performed with a troupe of street actors, and her father, the leader of that troupe, reluctantly helps Itto find her. Itto tracks Oyuki to a remote hot spring, where she is once again confronted by her former instructor, Enki. Wielding a sword that he sets on fire and possessing hypnotic eyes, Enki attempts to dominate her again, but Oyuki has broken his spell. She kills him by distracting him with her shocking tattoos. With her personal vengeance complete, Oyuki must now face the man hired to end her life. Itto respects her as a warrior, but he is bound by his contract, and he swiftly cuts her down, allowing her an honorable death.

Meanwhile, the master of the Yagyu clan, the silver-haired Retsudo, has been maneuvering politically to destroy Itto. He manipulates a local daimyo into summoning Itto, but Itto uses the baby cart’s arsenal of hidden blades and weapons to escape the daimyo’s compound, taking the nobleman hostage. As Itto retreats with his hostage, Yagyu marksmen kill the daimyo, and a massive force of Yagyu swordsmen descends upon the lone ronin. Telling Daigoro that he is entering the crossroads to hell, Itto launches himself into the attacking army, cutting a bloody path through the gulleys and rocky terrain. He battles Retsudo directly, stabbing him in the eye, but is himself wounded with a sword thrust into his back. Severely injured, Itto commands his young son to pull the blade from his flesh. He then places Daigoro back in the cart and slowly pushes it onward, continuing his eternal journey of revenge.