Manji is a notorious swordsman in feudal Japan, known as the "Hundred Man Killer" for having slain one hundred innocent men. Yet, something far more terrifying than his grim reputation exists: his immortality. This condition is the work of Yaobikuni, an eight-hundred-year-old nun who implanted bloodworms into Manji's body, granting him the ability to heal from nearly any wound. To atone for his crimes, Manji resolves to kill one thousand evil men. Yaobikuni accepts this proposal, stating that if he succeeds, she will lift his curse of immortality. Shortly after this promise, Manji encounters Rin Asano, a 16-year-old girl who asks for his help in killing those who murdered her parents. At first reluctant, Manji refuses Rin's desperate plea. However, recognizing her obvious lack of strength, he changes his mind and agrees to protect her for four years. With this partnership established, the two set out on a perilous journey of bloodshed, vengeance, and redemption, each seeking to fulfill their own life's purpose.