A family heads to a remote country home to celebrate the New Year with relatives. The gathering consists of parents Elaine and Jonah, along with their children. They are soon joined by Elaine's sister, Chloe, her husband Robbie, and their offspring. The initial excitement and festive cheer quickly dissipate as one of the children starts displaying violent behavior, inexplicably becoming ill. As the children interact, their behavior shifts from slightly disruptive to increasingly malevolent and dangerous.

The adults initially dismiss the bizarre conduct as typical childhood mischief or the result of some unknown sickness. However, the situation escalates dramatically when it becomes clear that something more sinister is amiss. Tensions rise and fears amplify as the children’s actions turn systematically violent, leaving the parents and remaining family members in a desperate struggle to comprehend the horrifying transformation occurring before them. The once joyous vacation dissolves into a nightmarish ordeal, leading the adults to an inevitable confrontation with their own primal instincts for survival.