Natsuki adores Natsu, who married her brother, as if he were her real sister, and whenever she has free time, she visits her brother and his wife's house. One day, while comforting Natsuki, who has broken up with her boyfriend and is heartbroken, Natsu kisses her impulsively. Natsu reveals that she is a lesbian who can only love women, and that her marriage to her brother was a sham, and invites Natsuki into a forbidden world. What will Natsuki's fate be?