In Wales, Jessica “Jess” Webb-Thomas, an American sculptor and former drug addict, meets with her estranged daughter Chloe (from Jess’s prior marriage and sent to foster care due to the former’s drug addiction) and invites her to come live with her and her second husband, Ben Thomas. Chloe declines. That night, she goes out with her friend Danny to a house where legend says a witch lived. The two knock twice on the door and leave. Danny starts experiencing paranormal occurrences and is later dragged away by an unseen force. When the demonic spirit frightens Chloe, she accepts Jess’ offer and moves in with her.
Chloe is initially hostile to her family. Strange events unfold, beginning when Chloe finds a human molar in her carrot and coriander soup. Jess suffers from a nightmare about an old woman crying in her house. In her nightmare, the woman looks at Jess and says “Przepraszam” (“I’m sorry” in Polish), before slitting her own throat. Jess tells Chloe about her dream and, finding this description familiar, Chloe tells her about Mary Aminov, a supposed witch that used to live next door. After her death by suicide, an urban legend started that she would come and get you if you knocked twice on her door. While sculpting, Jess’s model Tira is disturbed by Chloe’s presence and leaves. Chloe believes to be haunted by Baba Yaga, a demonic witch from Slavic folklore.