In a futuristic world dominated by cybernetic technology, ancient ninja souls are revived to wreak havoc in Neo-Saitama. Fujikido Kenji, a grieving salaryman who lost his family in a ninja conflict, is possessed by a mysterious ninja spirit after a near-death experience. Transformed into Ninja Slayer, he embarks on a relentless quest for vengeance, battling the malevolent Soukai Syndicate ninjas in the dark and dangerous Neo-Saitama.