“Sis, we’re each other’s only ‘sisters,’ right?” Ryu So-hee is a high school girl who was very sick as a child but has since recovered and now lives a seemingly normal life. She has a younger sister, Ryu A-young, who is still ill. A-young dotes on and clings to her older sister, but So-hee gradually begins to feel weighed down by this. After a certain incident, the relationship between the two starts to crack, and the once‑close sisters grow increasingly awkward and uncomfortable around each other. Within a family hiding dark secrets, a cruel mystery–thriller about two sisters unfolds.