Mizuki and her boyfriend Kazuya are soon to be married and are a happy couple, but Mizuki has a strong libido and is not satisfied with her boyfriend's sex, so she secretly masturbates. Kazuya, who has fulfilled his dream of studying abroad, invites Michael, a black man who helped him out, to his house, and the three of them start living together. After Mizuki accidentally sees Michael's huge penis, she can't stop thinking about it and crosses the line.