Manji, a highly skilled samurai, becomes cursed with immortality after a legendary battle in which he avenges the death of his sister. Haunted by his inability to die and the memories of those he has lost, Manji roams feudal Japan as a shadow of his former self. One day, he encounters Rin Asano, a young girl seeking vengeance for her murdered parents at the hands of the ruthless Ittō-ryū samurai school led by Anotsu Kagehisa. Moved by her plight and reminded of his sister, Manji reluctantly agrees to help Rin in her quest for revenge, becoming her bodyguard and protector.

As they delve deeper into their mission, Manji and Rin encounter numerous formidable foes from the Ittō-ryū, each with unique combat skills and deadly weapons. With each confrontation, Manji's extraordinary healing abilities are pushed to their limits, and he must rely on his centuries of combat experience to survive. Along the way, Rin's resolve and strength grow, transforming her from a helpless girl into a determined warrior. Their shared quest for vengeance strengthens their bond, forging an unbreakable alliance as they battle insurmountable odds to bring justice to those who wronged her family.