On Christmas Eve, suburban housewife Della Myers gets into an argument with her abusive husband Kenneth. After putting her two children to bed, she drives to the mall to buy some wrapping paper and cards. At the mall, she can't find a parking space for a while and angrily leaves a note on the window of a car that is parked using up two parking spaces. She leaves the store as the mall is closing, and the parking lot is nearly deserted. She notices the note is gone from the 'offending' car. As she enters her own car, the car on which she had left the note pulls up behind her.
She confronts the car, and four young men emerge—Huey, Vingh, and Tomás—led by Chuckie. They threaten to rape her. Della insults Chuckie, and a security guard intervenes, but he is shot dead by Chuckie. As the gang realizes that they have committed a murder, Della manages to start her car and drive away. They follow her, intending to kill her, as she is the only witness. As they pursue her some distance, she eventually crashes her car in a deserted area where homes are under construction. She takes a road flare and a toolbox out of her car and hides behind a back hoe.
Della runs through the buildings under construction to hide as they search for her. After some time, the thugs corner and threaten her by name, as they had found her drivers license in her purse in her car. As they have her open the toolbox, she hits Chuckie with a crescent wrench, and escapes again into some nearby woods. In the process of chasing her, Tomás accidentally steps on Huey, who falls through a construction site and dies from a broken neck.
After some hide and seek in the woods, Della beats and finally kills Tomás with a lug wrench. She flees through a creek, pursued by Chuckie and Vingh. Della sneaks up on Vingh and kills him with a screwdriver and hides behind a fallen tree. Chuckie tries to persuade Della to give up; he talks about her kids, saying that he is going to pay them a visit. He tells her what he thinks of her, that she lives a boring life she doesn't want, mistreated by her husband. He finds her, touches, and teases her face. She holds his hand, pulls him down and they kiss. He draws his weapon as they engage in foreplay. She tells him to have sex with her, and as he is distracted she ignites the road flare and blinds him, takes his weapon, and kills him.
Della returns home. Her husband Kenneth complains she was out late and is tracking mud throughout the house, but Della ignores him. She goes upstairs to check on her children who are both sleeping. The drunk Kenneth asks what she brought him from the mall, and she replies, "Nothing", and points the gun at him.

Question: Why does the gang want to kill Della?
Answer: She saw them murder the security guard
Alternative answer: For leaving an angry note on their car.