Su Na, once a bright and adorable girl, locks herself alone in her room after her best friend's suicide. She starts to talk and behave as if someone else exists in her room, but never opens the door to anyone. Finally, her uncle brings his fiance, Yun Mi, a psychologist specializing in hikikomori (people who isolate themselves from society), and she finds Su Na's trouble has something to do with the family's secret history.