Shulin (Lee Yoo-young), a high school teacher who is getting married soon, receives congratulatory gifts from everyone. After drinking the coffee on her desk, she immediately falls into a coma and is not awakened until the next morning by her students. From then on, her peaceful life is gone forever. Soon, Shulin's mobile phone receives a terrifying harassment message from the "Master". The content is a photo of herself when she was unconscious. What makes her even more horrified is that it seems that all this is a repeat of the same nightmare 14 years ago. Shulin finds former detective Guozhe (Kim Hee-won) to investigate the truth of the case together, hoping to find the mastermind behind the scenes. At this time, the students in Shulin's class also begin to receive harassing messages from the "Master", and the protagonist of the incident becomes a female student in the class. Guozhe, who has investigated similar cases, also feels that something is wrong. As the situation becomes more and more bizarre, what is the connection and involvement between the two cases? And who is the "Master" who is hiding in the dark and controlling everything?