In the midst of a snowy Christmas Eve, an affluent but lonely young boy named Thomas is left under the care of his elderly and ailing grandfather at their expansive family estate. Thomas, brimming with intelligence and fond of high-tech toys and video games, prepares to spend the night ensconced in his imaginative world. His mother, a businesswoman, is caught up in the chaos of her job, and she can only offer her son distant comfort through phone calls. With a home equipped like a fortress and filled with a variety of gizmos at his disposal, Thomas eagerly anticipates the nocturnal adventures he will have with his beloved dog, unsupervised and unrestricted by adult oversight.

However, the holiday reverie shatters when a psychotic criminal, disguised as Santa Claus, infiltrates the mansion with ill intent. The initial thrill of a visit from Santa quickly becomes a perilous game of cat and mouse as Thomas realizes the intruder's malevolent agenda. Using his wits, along with an assortment of gadgets and booby traps he's crafted, Thomas resolves to protect his home against the intruder. The sprawling house transforms into a battleground of cunning and survival, echoing the very video game scenarios Thomas cherishes. The lines between child's play and grim reality blur, and Thomas must confront the dangers that lurk in the shadows of his own home.