It's bad enough that boarding school students Samantha and Clara can't go home for the holidays, but things take a deadly turn when a murderous gang arrives on their doorstep - just in time for Christmas.
Clara and Samantha, two students at Blackvale Academy, an all-girls boarding school, have chosen to spend Christmas with their teacher, Rose, and her boyfriend, Jimmy. With a series of ritualistic killings inspired by a demon summoning ritual Maisie learned while attending Blackvale, Jude, Maisie, Grant, and Doug have been terrorizing the neighboring towns. The murderers are forced to seek safety in Blackvale after a fortuitous run-in with a police officer. There, they swiftly apprehend and take Rose and the others into custody, killing Jimmy in the process. They are irritated when the promised demon fails to appear, so they decide to kill Rose in order to complete the ritual at Blackwood.
Together with Jude, Maisie goes to the basement to look through the spell book from which she got the spell. There, she finds that she only got a portion of the ritual and that the ultimate sacrifice must be an unwholesome person. Clara, Samantha, and Grant are left alone when Doug pursues them. Jude behaves oddly and then leaves. As Maisie reads the spell, she understands that Clara, a demon, is supposed to be set free by the ritual. After that, we see Clara using her abilities on Grant to force him to release Samantha, telling her to leave her behind and run for her life. The demon then uses her abilities to take over Grant's mind, using him to seize Jude, Maisie, and Doug and make them unconscious. Samantha calls the police and escapes to a security station, but she decides to go back to save Clara.
When Maisie wakes up, she finds that Clara has left them naked in order to finish the ritual. Clara reveals that centuries ago, when the townspeople summoned her, they changed their minds and she was imprisoned inside the school walls. Clara makes them give up a number of personal things, which leads to Doug and Jude's deaths. After Maisie successfully overthrows Clara's hold on Grant, he commits suicide. When Samantha returns, she learns that Clara is the demon, which further breaks Clara's hold and releases Maisie, who starts to run in just her underwear. Samantha tells Clara, who is upset, that she could not leave a friend behind when she asks why she came back. They quarrel because Clara does not think anyone could be interested in her. She begs Samantha to kill Maisie and release her as she flees into the snow. Samantha obeys, and at the end of the movie, she and Clara depart to explore the world.
