Keisuke Iwasaki lives two lives. By day, he is a popular and muscular high school teacher, well-liked by his students. By night, he is the masked vigilante known as the Rapeman, who co-runs a business called Rapeman Services with his uncle, a former surgeon with ties to the yakuza. The services motto is righting wrongs through penetration.

Clients hire Rapeman Services to handle situations where conventional justice or social pressure has failed. These cases range from a jilted lover seeking revenge and parents wanting to discipline a rebellious daughter, to making disruptive coworkers more docile or creating a traumatic crisis to forge parental bonds. The manga is episodic, with each chapter presenting a new client and target. Although Keisuke occasionally expresses regret over the contracts he fulfills, he always completes the assigned task.

When on a mission, Keisuke dons a black leather ski mask shaped like the head of a penis and typically wears no trousers or underwear. He uses restraints such as handcuffs and adhesive sheets for control. If a victim becomes unresponsive or appears to be enjoying the violation, he applies special techniques intended to inflict more pain, including the M69 Screwdriver and the Infinite Loop. He prides himself on being a professional, has no tolerance for competitors, and possesses remarkable physical abilities, including climbing buildings with his bare hands and defeating armed yakuza members.

The series ran from 1985 to 1992, spanning 13 volumes. In the final episode, Keisuke is transferred to Osaka and declares that he will not continue his night business in the Kansai region, bringing the story to a close.