In feudal Japan, Manji, a feared swordsman known as the "Hundred Man Killer," has murdered one hundred innocent people. His infamy is compounded by his immortality, a curse placed upon him by Yaobikuni, an eight-hundred-year-old nun who implanted bloodworms in his body capable of healing nearly any wound.
To atone for his crimes, Manji pledges to kill one thousand evil men. Yaobikuni agrees to this arrangement, promising that his immortality will be lifted once he fulfills this oath. Shortly afterward, Manji encounters Rin Asano, a sixteen-year-old girl who begs him to avenge her parents' brutal deaths.
Initially, Manji refuses her request, but seeing her helplessness, he changes his mind and agrees to protect her for four years. Together, they embark on a perilous journey filled with violence, revenge, and the search for redemption, each driven by their own personal motives.