Mizuki and her boyfriend Kazuya are a happy couple on the verge of marriage, but Mizuki has a strong sex drive and finds Kazuya's sex unsatisfying, so she secretly masturbates. Kazuya invites Michael, a black man he befriended during his dream overseas study abroad, to live with them, starting their life together. After accidentally seeing Michael's huge dick, Mizuki can't stop thinking about it and ends up crossing the line.
