A parasitic organism wreaks havoc on All-Mart employee Monty while he is busy prepping the store for the bustling Black Friday sale. Soon, Monty undergoes a horrifying transformation into a monster, subsequently attacking two of his co-workers. Meanwhile, Ken Bates [Devon Sawa], burdened with the responsibility of working on Thanksgiving, drops off his daughters, Lyla and Gracie, for dinner with their mother and her new spouse, Grant. He then picks up his colleague Chris Godecki and together they head to their late shift at We Love Toys.
As anxious shoppers gather outside, Ken and Chris join their fellow employees, including Marnie, Brian, Archie, Ruth, Emmett, Anita, and Bircher, in getting the store ready for the anticipated rush. In an unfortunate turn of events, Bircher accidentally locks himself outside while taking a smoking break. Perilously transformed by the parasite, a frenzied shopper turns on Bircher, leading him to morph into a creature himself. As the store manager, Jonathan Wexler instructs his team to open the doors, shoppers flood in, but the excitement quickly turns into chaos as the parasitic infection spreads, transforming the patrons into violent predators.
Amidst the pandemonium, Emmett becomes infected and violently spews a tentacle that kills Anita. As he becomes a creature, he dashes toward the Santa's Village display. Ruth takes refuge in the office, while Archie spearheads the effort to guide Chris and Brian through hordes of mutants to secure an exit door before more infected shoppers can surge inside. Meanwhile, Ken and Jonathan confront a creature in the restroom.
In a shocking revelation, Marnie sneaks into Santa's Village and discovers that the mutated shoppers are being absorbed into a growing mass. After successfully securing the exit door, Jonathan, Ken, Chris, Marnie, Brian, Archie, and Ruth converge back on the shop floor. Suddenly, a possessed officer drives a speeding police car that crashes through the storefront window, tragically running over Ruth. The mutants continue to storm inside. Just as Ken rescues Chris, one of the monsters takes down Archie, forcing Ken, Marnie, Chris, Brian, and Jonathan to barricade themselves within the storeroom.
During a moment of bonding over sliced turkey, the group exchanges stories about their experiences at the store, but tensions eventually fray as Ken and Chris clash over their unsatisfying lives while Marnie confronts Ken about their lack of a real romance. The situation escalates when a mutant bursts into the storeroom; Ken gets bitten in the melee, leading him to fear he will become infected. Making the tough decision, he opts to remain behind while Chris, Marnie, Jonathan, and Brian head to the loading dock in search of a truck for their escape.
As they attempt to start the truck, they are interrupted by another mutant attack, forcing them to flee to the roof. Just then, disaster strikes as the building ignites, and the main sac keeps expanding into a colossal monster that begins to consume the whole store. In a noble sacrifice, Jonathan offers himself to the horde, allowing Chris, Marnie, and Brian a chance to escape. As the chaos unfolds, Brian reveals a startling truth: he inadvertently bit Ken in the earlier skirmish, indicating that Ken is not actually infected.
As the monstrous entity swallows up all the mutated shoppers to grow even bigger, it breaks through the store's roof, leading to a deadly confrontation where it kills Brian. Reuniting outside, Ken distracts the creature, enabling Chris to devise a plan that tricks the monster into ingesting a forklift, resulting in a massive collapse as the creature succumbs to the flames.
In the end, Ken, Chris, and Marnie make their escape in Ken's car, but looming in the distance, they spot more monstrous figures emerging from other stores, hinting that the nightmare may just be beginning.
