Yuna, the younger sister who has secretly harbored feelings for her older brother, gradually begins acting more provocatively to turn her emotions into action. Unable to resist her seduction any longer, the brother crosses the forbidden line into incest. With her long-held desire finally fulfilled, Yuna starts demanding sexual encounters from her brother every single day. One day, their parents leave home and won't return until late at night. Seizing the opportunity, Yuna calls out to her brother, making plans to spend the entire day alone together indulging in sex. "Let's make love all day long today, big brother," she passionately urges.