At Blackvale Academy, an esteemed all-girls boarding school, the holiday season takes a dark turn as two students, Clara and Samantha, find themselves remaining on campus for Christmas alongside their teacher Rose and her boyfriend Jimmy. As the festivities begin, a malevolent group of murderers, including Jude, Maisie, Grant, and Doug, have been wreaking havoc in the nearby towns with a chilling series of ritualistic killings, inspired by a demon summoning ritual that Maisie stumbled upon while enrolled at Blackvale.
A fateful encounter with a police officer forces these unscrupulous killers into the already tense atmosphere of Blackvale, where they swiftly overpower the group, capturing Rose and the others while tragically ending Jimmy's life. Their sinister intent is to complete the ritual within the school; however, they grow increasingly frustrated when the anticipated demon fails to make an appearance.
In a twist of fate, Maisie descends into the basement with Jude to unearth more about the spellbook from which she had drawn the ritual. In the depths of their investigation, she realizes that she had only uncovered part of the necessary incantation and that the final offering must be someone pure of heart. Meanwhile, Doug pursues them, leaving Clara and Samantha vulnerable in the grip of Grant's menace. As Jude's odd behavior raises alarms, Maisie begins reciting the spell, triggering revelations about its purpose: it is designed to unleash a demon named Clara.
Empowered, Clara uses her unearthly abilities on Grant, compelling him to liberate Samantha, instructing her to escape while leaving Clara behind. Meanwhile, the demon’s powers take hold of Grant, turning him into a puppet to subdue Maisie, Jude, and Doug. Desperate to signal for help, Samantha manages to reach a security station, where she contacts the police but makes the selfless decision to return to save Clara.
As the tension mounts, Maisie regains consciousness, only to find that Clara has taken a sinister turn, imprisoning them without clothing in order to fulfill the ritualistic demands. Clara reveals her tortured history—she had been contained within the school's walls ages ago after the townsfolk had a change of heart mid-ritual. The torment escalates as Clara forces the girls to embrace painful personal sacrifices, leading to a series of tragic outcomes, including the demise of Doug and Jude.
In one final act of rebellion, Maisie breaks free from Clara's control over Grant, who tragically ends his own life. Samantha, ever determined, returns and boldly confronts Clara, unveiling her true nature as a demon. This revelation sparks a fierce conflict between them, as Clara questions the very notion of loyalty and friendship, demanding Samantha to prove her worth by eliminating Maisie during her flight through the wintry landscape. The chilling narrative concludes with an emotional pivot as Samantha reaffirms her commitment to her friend, thus embarking on a journey filled with uncertainty and camaraderie as they prepare to traverse the world together, leaving behind the darkness of Blackvale Academy.
