Baishun group: Kokujô suru hitozuma (1979)

Shinichi is a humble hand towel delivery man. One day, he bumps into a beautiful woman and picks up a red notebook. To his surprise, it contains a detailed record of married women's prostitution activities. Seeing his chance, Shinichi blackmails the married women with the notebook and has sex with them one after another. However, he can't seem to meet the beautiful woman who dropped the notebook...