Sayuri is a housewife living with her daughter and son-in-law in a two-family home. Occasionally, she takes on swimsuit modeling jobs introduced by acquaintances. One day, her son-in-law Kunio begins masturbating after seeing her in a swimsuit. It turns out he's hung with a huge cock, causing difficulties in his marital life. But Sayuri, captivated by his massive size, convinces herself it's for her daughter's sake—and her own—and relentlessly seduces him day and night. Enthralled by her seductive, lustful behavior, Kunio, starved for intimacy, eventually finds himself unable to resist.