The story takes place before the events of the Kojiro of the Fuma series and focuses on Musashi Asuka's past. As a child, to save his sister who was sick and dying, Musashi Asuka is saved by a mercenary and gives up his life in exchange for his sister's salvation. So that one day they can meet again, Musashi must become a Ninja Hunter. However, until this is accomplished, the path will be turbulent and unknown.