One day when he was considering marriage with his girlfriend, who had been living together for some time, her younger sister, Ria-chan, came to stay overnight. Although she was somewhat naive, Ria-chan was an incredibly innocent and pure beauty unlike anything he had seen before. What's more, she was extremely easygoing and allowed him to have his way with her since she didn't resist much at all! To his surprise, after secretly having his way with her one night, she suddenly became obsessed with him and demanded sex every day, leaving him unable to refuse her advances!
