Mizuki and her boyfriend Kazuya are a happy couple about to get married, but Mizuki has a strong sex drive 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 begin living together. After Mizuki accidentally sees Michael's huge penis, she becomes obsessed with it and crosses the line.