Yasuko Yagami ONANIE (1985)

Mio lives alone in a villa in the woods near a lake. Her ex-husband Muraoka comes to visit her and demands that they get back together. However, Mio enters her room and locks the door. After a while, she hears moaning from inside the room. She goes around to the balcony and peeks in, and finds Mio leaning her body out and engrossed in masturbating...The next day, students Mako and Toru come by car. Toru has been told that he has a part-time job, but he is not told what it entails. That night, Muraoka, who is still there, is shocked to see Mio and Mako entangled. He then tries to get Toru, a virgin, to embrace Mako and break up with Mio, but..