When a salaryman's wife and child are slain in a ninja war, he fuses with a supernatural entity to take vengeance on all ninjas.

After his wife and child are killed in a ninja turf war, salaryman Kenji Fujikido gets possessed by a mysterious ninja soul known as Naraku Ninja. As Naraku's craving for massacring the evil ninja overlaps with Fujikido's lust for revenge, together they bring Ninja Slayer - a terrifying ninja-killing grim reaper - into existence.