Mizuki and her fiancé Kazuya are a happy couple on the verge of marriage, but Mizuki has a strong sex drive and isn't satisfied with Kazuya's lovemaking, secretly resorting to masturbation. Kazuya returns home after completing his dream of studying abroad and invites Michael, a black man to whom he owes a debt of gratitude, to live with them, beginning a new life as a trio. After accidentally seeing Michael's huge cock, Mizuki becomes obsessed, eventually crossing the line with him.