Private detective Sayaka Hattori, a descendant of the Iga ninja clan, fights against evil as a modern-day ninja detective. One day, a man named Kudo hires her to investigate his son, who, despite being unemployed and living as a NEET, has suddenly begun engaging in antisocial behavior and is secretly possessing a Japanese sword. Sensing a "stench of sin," Sayaka immediately launches an investigation and discovers the hideout of the Japan Overthrow Party, a cult secretly brainwashing the boy. Using her ninja skills, she infiltrates the base, uncovers evidence linking them to the recent series of grotesque serial murders, and rescues the son. However, her actions draw the party's deadly attention. Determined to confront the organization, Sayaka sneaks into their hideout once more, only to be attacked by Kudo, now brainwashed by the cult. Falling victim to a treacherous trap, she is cursed by a demonic technique called "Inmu," transforming her into an insatiable nymphomaniac. Struggling to resist overwhelming sexual urges with her remaining sanity, Sayaka gradually succumbs to the party members' relentless, sadistic torment. Can she save Kudo and destroy the Japan Overthrow Party before losing herself completely?