In ancient japan, ninjas were known as skilled warriors who ruled with their karate during a time of peace. however, some of them committed an illegal form of hara-kiri and stored their souls at kinkaku temple for future resurrection. their history was falsified and hidden, leading to the eventual forgetting of the truth about these ninjas.
in the present day, with the advancement of cybernetic technology and electronic networks, sinister ninja souls have been revived from thousands of years ago and are now wreaking havoc in the shadows of neo-saitama. fujikido kenji, a salaryman whose family was killed in a ninja war, is possessed by an enigmatic ninja soul and becomes ninja slayer, determined to cheat death and take on the soukai syndicate ninjas in mortal combat.
meanwhile, a grim reaper destined to kill evil ninjas embarks on a personal vengeance war in the dystopian underworld of neo-saitama.