A few years after losing her beloved husband, Nobuko now lives with her daughter and son-in-law. It seemed as though their harmonious relationship would continue as is, but her son-in-law, Yukio, accidentally sees Nobuko masturbating. From that day on, Yukio has never been able to forget Nobuko's beautiful and seductive lewd behavior... One day, when the two of them happen to be alone on a holiday, seeing Nobuko's back, he has a flashback to that day's lewd behavior. Although he knows it's wrong, Yukio can't contain his excitement and throws away all reason...