Suffering from sexual frustration due to a lack of intimacy with her husband, Mariko Sata spends her days restlessly searching for ways to satisfy herself. One day, she discovers that her son-in-law, Shinsuke, who lives with her, is secretly masturbating for the same reason. By chance, she catches sight of Shinsuke's abnormally large cock, and is instantly captivated. Convincing herself that she's helping relieve his pent-up desires on her daughter's behalf, Mariko begins relentlessly seducing Shinsuke day and night. Unable to resist his stepmother's alluring charm, Shinsuke gradually gives in to her advances, letting her tease and pleasure his throbbing cock again and again.