In Baby Cart at the River Styx, Ogami Itto and his young son Daigoro are hired by the Kuroda clan to assassinate Oyuki, a female leader of a group of ninja warriors known as the Sirai. However, Itto discovers that the Sirai have been hired by the Yagyu clan to stop him from completing his mission.
As he navigates through various traps and obstacles set by the Sirai, Itto also has to protect his son, who is often left alone in dangerous situations. Meanwhile, Itto is informed by his ally, a woman named Omatsu, that a Yagyu spy named Bizen has sold out the Kuroda clan's secrets to the Shogunate and is planning to flee the country.
Itto decides to take on both missions at once, first confronting the Sirai and then pursuing Bizen. In a stunning battle, Itto and Daigoro face off against Oyuki and her fellow Sirai, who use a variety of ninja weapons and tactics to try and take them down.
In addition to the action-packed set pieces, Baby Cart at the River Styx also explores themes of honor, loyalty, and betrayal, as characters struggle with their allegiances and motivations. Overall, it is a thrilling and emotional entry in the Lone Wolf and Cub franchise.