It's Christmas time for one particular family, which means yet another gathering at the home of workaholic Tom and OCD Sarah, and the rest of the dysfunctional unit for three claustrophobic days that mostly involve snippy complaining and petty squabbling. Tom and Sarah's young son Max, at perhaps the last age when Santa Claus might still be seemingly real idea, writes a letter to the mythical North Pole resident that relates his wish of Christmas with his family being "like it used to be." However, when his letter is found by his bratty cousins and openly mocked, Max tears up his letter, and abandons his wishful beliefs, which inadvertently causes a chain of horrific events when Krampus, the evil shadow of Saint Nicholas who terrorizes the naughty children, arrives in the wake of a massive blizzard to put an end to the family bickering for good.