Mizuki and her boyfriend Kazuya, who are about to get married, are a happy couple, but Mizuki has a strong sexual desire and secretly masturbates because her boyfriend's sex life is not up to par. Kazuya, who has always dreamed of studying abroad, invites Michael, a black man who has been taking care of him, to his house and the three of them begin living together. After Mizuki accidentally sees Michael's big dick, she becomes so worried about it that she goes overboard.