Manji, a legendary swordsman from Japan's feudal era, is infamous as the "Hundred Man Killer" for slaying a hundred innocent men. Yet, even more terrifying than his dark reputation is the fact that he cannot die. This is due to the actions of Yaobikuni, an eight-hundred-year-old nun, who implanted bloodworms in his body that can heal nearly any injury.
Seeking to make amends for his past sins, Manji vows to kill one thousand evil men. Yaobikuni accepts this plan, promising to lift his immortality curse if he succeeds. Shortly after, Manji encounters Rin Asano, a sixteen-year-old girl who asks for his help in avenging her parents' murder.
At first, Manji turns down Rin's desperate request. But seeing her clear lack of strength, he changes his mind and agrees to guard her for four years. With this pact sealed, the pair sets out on a dangerous journey filled with bloodshed, revenge, and redemption, each pursuing their own personal mission.