In a world where cybernetic technology reigns supreme, ancient ninja souls are resurrected to wreak havoc in Neo-Saitama. Fujikido Kenji, a grieving salaryman, becomes possessed by a vengeful ninja spirit after a tragic loss. Transformed into Ninja Slayer, he embarks on a relentless quest to eliminate evil ninjas, leading to intense battles against the ruthless Soukai Syndicate in the dark dystopian realm of Neo-Saitama.