The disgraced former executioner, or Kogi Kaishakunin to the shogun, Ogami Ittō is now living rough on the land with his three-year-old son Daigoro, travelling the countryside as a hired killer. Pushing his son in a baby cart, he stops at a bathhouse looking for a room and a bath, and is eagerly welcomed in by a young woman. However, the manager of the house sees Ittō as a dirty vagabond and scolds the young woman for letting him in the house. Overhearing this, Ittō goes to the baby cart and retrieves a bundle and hands it to the manager for safe keeping – it is 500 gold pieces, earned from a recent job. The manager's tone quickly changes, but when he tries to wash Daigoro's feet, the boy kicks water at the old man and tromps across the floor, leaving wet footprints.
Ittō's activities are being watched by the Kurokawa spy clan of shinobi-class ninja, which have fallen in with Ittō's nemesis, the Shadow Yagyū. They report on his activities to Sayaka, head of Akari Yagyū clan of female assassins. But the Kurokawa are unsure that the women are up to the task of killing Ittō. Sayaka laughs confidently and tells the Kurokawas' leader to send their best man into the room. She then orders the man to try to exit the room. He tries to do so by grappling onto the ceiling, but the female assassins set upon him and make short work of him, hacking off his ears, fingers, arms and legs, leaving the man a writhing heap of just a torso and head before he is finally finished off.
Ittō, meanwhile, is hired by a clan that specializes in making indigo dye and has a secret process. One of the clan plans to sell out the secret to the shogun. Ittō must kill him. The turncoat will be escorted by the three Hidari brothers, each a master of a deadly weapon – the iron claw, the flying mace and a pair of armored gloves.
As he travels to his job, Ittō encounters three groups of female assassins. The first group is disguised as an acrobat troupe that turns deadly as their gymnastic moves are combined with blades and turned against Ittō. He then encounters a pair of women who with blades on their straw hats that they throw at Ittō as if they are Frisbees. Finally, some women washing vegetables by the river turn out to be assassins and the daikons they are washing are wielded as weapons. Ittō kills them all.
Then he meets Sayaka, who catches him, his son and the baby cart in a steel net. Ittō cuts his way out of the net and engages in a sword duel with her. He delivers what should be a disabling blow to her ankles, but the woman fighter jumps straight up out of her kimono to reveal a fishnet body suit and then runs away, bizarrely jogging backwards.
Next, the Kurokawa clan are waiting for Ittō, who puts together his naginata (disguised as railing on the baby cart) and gives the baby cart a shove toward the waiting enemies. Daigoro, still in the baby cart then activates blades in the axles of the cart, which cut off the feet of several men. The battle that ensues between Ittō and the Kurokawas is fierce and Ittō is injured before he has killed them all.
Weary from the endless fighting, Ittō struggles along the road and eventually finds shelter in a shack. Daigoro, seeing that his father needs his help, must do what he can. Unable to carry water in his tiny hands, Daigoro carries water from the river in the only vessel available – his mouth. He spits the few drops he could carry between his father's parched lips. For food, Daigoro finds some rice cakes given as an offering to a Buddha statue and takes them for his father, leaving his vest in exchange.
Ittō recovers and finds that his son is missing. Daigoro has been taken by the Kurokawa and Sayaka, tied up and suspended over a water well. If Ittō attacks, they will let go of the rope and Daigoro will plunge to his death. Daigoro lets his sandal drop into the well, giving Ittō a gauge of how deep it is. He then makes his move as the rope unspools, stopping it just in time.
Sayaka watches silently and makes no move to engage the swordsman, realising his technique far outstrips her own and perhaps also out of a sense of honor for the devotion of the father to his child.
Ittō then finds himself aboard a ship, carrying the three Hidari brothers. Ittō is met by several other members of the indigo-dye clan, who try to kill the Hidaris but are bloodily rebuffed. The clan then sets the ship on fire in an attempt to kill the Hidaris, but the three killers escape. Ittō tosses Daigoro and the baby cart into the water, the cart proves itself by floating.
Sayaka has followed Ittō onto the ship and in the water, she tries to kill him, but is disarmed. Ittō, with Daigoro and Sayaka, find shelter, but they are cold. Ittō disrobes himself and Daigoro, then turns on Sayaka, tearing off her clothes. But he does not mean to rape her. Instead, he is seeking to get her out of her cold, wet clothes and cling to her while nude. "Three people are warmer than two," he explains. She thinks about taking his sword and killing him, but the cozy scene, with Daigoro sitting between them, playfully fondling her breast (and Itto's), makes her abandon the plan.
The final showdown takes place on a vast area of sand dunes. The Hidari brothers are at the head of a caravan of men carrying a palanquin with the indigo expert inside. The brother with the iron claw runs forward and thrusts his claw into the sand, which boils up with blood. There are men hiding in the sand. He digs his claw into the sand several times, each time creating a pool of blood and pulling up a hiding warrior by his head. The rest of the hidden men in the sand emerge and fight, but the Hidari brothers dispatch them all.
Ittō awaits, alone, at the top of a large dune. Each brother is dispatched in a high-pressure spray of blood, with the last brother dispatched in a lethal stroke along the throat, a cut that sprays blood in a fine mist, while making a sound like the "howling of the wind". The final slain Hidari brother comments such a fabled finishing stroke is referred to as "Mogaribue", and wishes he had heard the sound just once from the many people he has killed, but is instead hearing it from his own neck as his life slowly drains away.
Ittō approaches the palanquin with the traitorous indigo expert inside, quickly finishing him before gathering Daigoro and again setting off. The end sees them out of the desert and on a coastal trail, followed by Sayaka. Aware of her presence Ittō stops the cart, looking straight ahead whilst holding out his dotanuki blade, as Sayaka wielding a katana is revealed behind him. Ittō stands ominously still until he hears the sound of Sayaka dropping her sword; knowing that she can never defeat the master swordsman.Tomisaburo Wakayama as Ogami Ittō
Akihiro Tomikawa as Ogami Daigoro
Kayo Matsuo as Yagyu Sayaka
Akiji Kobayashi as Ozuno, Leader of the Kurokuwa Group
Minoru Ohki as Tenma Hidari, Master of Death 2
Shin Kishida as Kuruma Hidari, Master of Death 3
Lone Wolf and Cub: Baby Cart at the River Styx was released theatrically in Japan on 22 April 1972 where it was distributed by Toho. An English-language dubbed version of the film was released by New World Pictures on November 11, 1980. The film is heavily altered with a 90 minute running time.
The film was released on home video in the United States in its original form in Japanese with English subtitles as Lone Wolf and Cub - Baby Cart at the River Styx by Samurai Cinema, a division of AnimEigo Inc. The 1980 version was released to home video by AnimEigo in 2006.Most of this film, along with 12 minutes of footage from Lone Wolf and Cub: Sword of Vengeance was used to make Shogun Assassin, a 1980 film released as an English-language compilation of the Baby Cart series for American audiences.