Hee-yeon (played by Yeom Jung-ah)'s son Jun-seo is lost in Seoul. Heartbroken, she takes her amnesiac mother-in-law back to her hometown Changshan to recuperate, hoping that her mother-in-law can remember how Jun-seo disappeared as soon as possible. One day, Hee-yeon meets a mysterious girl in ragged clothes in a cave near Changshan. Perhaps out of sympathy or loneliness, Hee-yeon takes the girl home, intending to take care of her before finding the nameless girl's parents. The two get closer and closer, and gradually the girl becomes like Hee-yeon's daughter. She calls Hee-yeon "Mom" and calls herself "Jun-hee". Her appearance and voice are more and more like the disappeared "Jun-seo". After the mysterious girl arrives, incomprehensible strange things happen one after another. Hee-yeon's mother-in-law first hears hallucinations, and then is abducted by the monster that is said to live in the cave of Changshan... Can Hee-yeon find her son Jun-seo? Can the broken family be reunited again under the shadow of the monster?