A parasitic organism attacks All-Mart employee Monty as he prepares the store for its Black Friday sale. Monty mutates into a monster and attacks two co-workers. Because he has to work on Thanksgiving, Ken Bates drops off his daughters Lyla and Gracie for dinner with their mother and her new husband Grant. Ken then picks up his co-worker Chris Godecki and drives to their late shift at We Love Toys. While anxious shoppers assemble outside, Ken and Chris join co-workers Marnie, Brian, Archie, Ruth, Emmett, Anita, and Bircher as they get ready to open the store for Black Friday. Bircher accidentally locks himself outside the building while smoking a cigarette. Possessed by a parasite, a rabid shopper mauls Bircher, causing him to transform into a creature too. Store manager Jonathan Wexler has his employees open the doors. Shoppers pour in, but they gradually turn violent as the parasitic mutation spreads. Everyone eventually realizes that the shoppers are transforming into murderous creatures as they are attacked and forced to fight back. Possessed by a parasite, Emmett emits a tentacle that kills Anita. Emmett turns into a creature before fleeing toward the store's Santa's Village display. Ruth holes up in the office while Archie leads Chris and Brian through a mutant horde to close an exit door before more shoppers get inside. Jonathan and Ken battle a creature in the bathroom. Marnie sneaks into Santa's Village and sees that transformed shoppers are being assimilated into a growing sac. Once the exit door is secured, Jonathan, Ken, Chris, Marnie, Brian, Archie, and Ruth regroup on the store floor. Driven by a possessed officer, a speeding police car crashes through the front window. The car runs over Ruth. Mutating shoppers begin pouring inside again. After he rescues Chris, one of the monsters kills Archie. Ken, Marnie, Chris, Brian, and Jonathan barricade themselves in the storeroom. While talking about their various tenures at the store, the co-workers bond over sliced turkey meat. However, the group conversation turns confrontational when Ken and Chris argue about their disappointing stations in life and Marnie calls out Ken for not having a real romance with her. A mutant breaches the storeroom. Ken gets bitten in the commotion. Presuming he is now infected and will soon transform, Ken stays behind while Chris, Marnie, Jonathan, and Brian go to the loading dock to obtain a truck for escaping. With Chris unable to start the truck, he and the others fight off another mutant before making their way to the roof. Meanwhile, the building catches fire as the main sac continues growing into a massive monster that soon fills the entire store. Jonathan sacrifices himself to a horde so Chris, Marnie, and Brian can escape back down to the ground. Brian confesses he bit Ken in the earlier commotion, so Ken is not actually infected. Having assimilated all of the mutated shoppers, the massive monster breaks through the store's roof. Brian tries to confront the creature, but it kills him. Ken re-joins Chris and Marnie outside. Ken acts as a distraction while Chris tricks the monster into swallowing a forklift, which causes the injured creature to collapse into the fire. Ken, Chris, and Marnie escape in Ken's car. However, more massive monsters are seen emerging from other stores in the distance.