The shogun's illegitimate son decapitates a peasant which draws in nomadic Nemuri Kyoshiro into more sword-fighting adventures when he's blamed for the beheading.
After years on the road establishing his reputation as Japan's greatest fencer, Takezo returns to Kyoto. Otsu waits for him, yet he has come not for her but to challenge the leader of the region's finest school of fencing. To prove his valor and skill, he walks deliberately into ambushes set up by the school's followers.
A sadistic Daimyo (feudal lord) rapes a woman and murders both her and her husband, but even when one of his own vassals commits suicide to bring attention to the crime, the matter is quickly hushed up. Not only will there be no punishment, but because the Daimyo is the Shogun's younger brother, he will soon be appointed to a high political position from which he could wreak even more havoc. Convinced that the fate of the Shogunate hangs in the balance, a plot is hatched to assassinate the Daimyo. The two most brilliant strategic minds of their generation find themselves pitted against each other; one is tasked to defend a man he despises, and has a small army at his disposal. The other is given a suicide mission, and has 12 brave men. They are the 13 Assassins.
Fearless Edo-period police inspector Hanzo Itami, nicknamed The Razor, has developed his own unique way of extracting information for his inquiries. His first adventure sees him investigating his superior officer's mistress, whom he suspects of having ties with a reputed criminal on the loose.
Following the death of the second Tokugawa shogun, it is revealed that he was poisoned by retainers of his son Iemitsu in hopes of gaining him the shogunate despite the stammer and birthmark which undermine his respect. Iemitsu and his brother Tadanaga become bitter rivals for the shogunate, and the land is split into factions, eventually erupting into warfare. Iemitsu's mentor, his fencing instructor Yagyu, is fixated upon securing Iemitsu the shogunate and ends up betraying everyone, even his own family, in pursuit of the goal.
Ryunosuke, a gifted swordsman plying his trade during the turbulent final days of Shogunate rule, has no moral code and kills without remorse. It’s a way of life that leads to madness.