Yamoto Koki, a girl possessed by the Ninja Soul of Shi Ninja, uses her powers to save herself and her friend Asari from a gang attack. Shogo, a transfer student, is bailed out of jail and given an ultimatum.
Kenji Fujikido is a salaryman whose wife and child were killed in a ninja turf war. In a brush with his own death, Fujikido is possessed by an enigmatic ninja soul known as Naraku Ninja. Fujikido cheats death and becomes “Ninja Slayer” - a Grim Reaper destined to kill evil ninja, committed to a personal war of vengeance. Set in the dystopian underworld of Neo-Saitama, Ninja Slayer takes on Soukai Syndicate ninja in mortal combat.