Elaine, her second husband husband Jonah, her teenage daughter Casey and her children Miranda and Paulie travel to the isolated house of her sister Chloe to spend the New Year with her family. They are welcomed by Chloe, her husband Robbie and their children Nicky and Leah. Casey is upset since she will miss a party and Paulie vomits, but his parents believe he is sick because of the travel. During the night, Nicky and Leah become also sick and Leah vomits something strange. Along the day, the other children become also infected and Miranda attacks Chloe. Robbie brings the other children to play in the snow and while riding in a sledge, Nicky puts some tools in the way and Robbie dies with the impact on his head. Soon the children attack the adults and Casey and her mother seeks shelter. Will someone survive?