After Tsubaki's husband is sent away on a long-term work assignment, she moves in with her daughter and son-in-law. One night, she catches Takeshi masturbating after his wife refuses sex because his cock is simply too big. Having gone without sex herself, Tsubaki is captivated by the sight of his massive erection. Convincing herself that her daughter's failure is somehow her responsibility, she relentlessly seduces him day and night. Eventually Takeshi can no longer resist; fully aware that she is his mother-in-law, he eagerly gives in to her filthy invitation.