After his wife and child are killed in a ninja turfwar, salaryman Fujikido Kenji gets possessed by a mysterious ninja soul known as Naraku Ninja. As Naraku's craving for massacring ninja overlaps with Fujikido's lust for revenge, together they bring Ninja Slayer, a terrifying ninja killing Grim Reaper, into existence. The more their mental resonance and physical symbiosis deepens, the more powerful Ninja Slayer's karate becomes. If Fujikido completely surrenders himself to Naraku, the strength of his karate will become nearly invincible.