In the Edo period of Japan, Isaburo Sasahara is a vassal of the daimyo of the Aisu clan. Sasahara is the most skilled swordsman in the land, whose only rival in ability is his good friend Tatewaki Asano. Isaburo is in a loveless marriage with a shrew of a woman. One day one of the daimyo's advisors orders Isaburo's elder son Yogoro (Takeshi "Go" Kato) to marry the daimyo's ex-concubine, Ichi, even though she is the mother to one of the daimyo's sons. With much trepidation, the family agrees. In time, Ichi and Yogoro find love and happiness in the marriage and a daughter Tomi is born.
In the meantime, the daimyo's primary heir dies, and he orders his ex-concubine to rejoin his household to care for their son and heir.