Living with his girlfriend, he was starting to consider marriage. One day, her sister, Riya-chan, came to stay over. Though a bit naive, Riya-chan was a beautiful and innocent girl, unlike her sister. She was easily persuaded and a "pussy-licking" type, making her easy to have sex with. Secretly sleeping with her while keeping it from his girlfriend, she unexpectedly awakened to her erotic side and started demanding sex daily, turning the tables on him.