In Edo period Japan, Ogami Itto serves as the Kogi Kaishakunin, the shogunate’s official executioner, a high-ranking position of ritual importance. His world collapses when the Yagyu clan, a powerful shadow organization controlling the shogun’s spies and assassins, frames him for treason to claim his post for themselves. Following a botched attempt to eliminate him, Yagyu assassins murder Itto’s wife, Asami, leaving him alone with their infant son, Daigoro.

Refusing to accept death, Itto forces the Yagyu leader Retsudo to allow him a duel against the clan’s finest swordsman with the condition that, if he wins, he and his son will be permitted to become masterless ronin. During the duel, Itto straps a mirror to Daigoro’s forehead, using the reflected sunlight to blind his opponent before beheading him. Disgraced and stripped of his title, Itto now wanders the countryside pushing a wooden baby cart that carries Daigoro and conceals an arsenal of hidden weapons. He becomes an assassin for hire, hanging a banner on his back that reads Ogami: Sword for Hire, Son for Hire. He has given Daigoro a choice between a child’s toy ball, which would have meant death to join his mother in the afterlife, or a sword, representing the path of demons and damnation. The infant reached for the sword.

As Itto travels, he accepts a commission from a rural clan’s chamberlain to eliminate a group of conspirators plotting to assassinate their lord. The targets have taken refuge in a remote mountain village that houses a hot spring spa. Upon arrival, Itto finds the village overrun by a gang of violent ronin hired by the conspirators. He is disarmed and taken hostage along with several travelers, including a prostitute named Osen. The ronin leaders mock Itto and, to humiliate him, order him to have sex with Osen while they watch. Having embraced the meifumado, the spiritual way of demons and damnation, Itto has abandoned conventional honor and complies, an act that both startles and moves Osen.

When the conspirators finally arrive to meet the ronin, Itto reveals his true identity. He unleashes the hidden arsenal within his baby cart, including a naginata and throwing blades, and single-handedly slaughters the ronin in a brutal, one-sided massacre. One conspirator fires twin matchlock pistols at him, but Itto uses the cart’s armored underside as a shield before leaping over it to split the man’s skull with his sword. He kills the final fleeing conspirator by slicing open his chest. Osen, who has developed feelings for the stoic assassin and his son, attempts to follow him as he leaves the village. Itto severs the ropes of a bridge behind him, preventing her from joining him on his road to hell. He then continues his journey, pushing Daigoro in the cart as they wander toward their next contract and, ultimately, toward the final confrontation with the Yagyu clan that destroyed his life.