Mizuki and her boyfriend Kazuya, who are about to get married, are a happy couple, but Mizuki has a strong libido and secretly masturbates because her boyfriend's sex is unsatisfactory. Kazuya, who had dreamed of studying abroad, invited Michael, a black man who took care of him, to his house, and the three of them started living together. After Mizuki happened to see Michael's big cock, she became so obsessed with it that she crossed the line.