Natsuki deeply admires Natsu, who married her older brother, regarding her like a real sister, and frequently visits the couple's home whenever she has free time. One day, while Natsuki was heartbroken after breaking up with her boyfriend, Natsu approached to comfort her—and impulsively kissed her. In that moment, Natsu confessed she is a lesbian who can only love women, revealing her marriage to Natsuki's brother was merely a sham. Then, she draws Natsuki into a forbidden world. What follows is how their bond intensifies and how Natsuki's fate unfolds—this is where the story truly begins.