The film begins with a CIA internal memo being typed across the screen. It states that a man named Arlington Steward has recovered from severe burn wounds and is delivering units related to the Mars project.



The film then opens up in 1976 Richmond, Virginia with Norma and Arthur Lewis cuddling in bed after a night of passionate lovemaking (this scene has been removed from screenings in North America to obtain a PG 13 rating), and awaking at 5:45 am as the doorbell rings. Norma goes downstairs and looks through the peephole seeing a black car drive off. Upon opening the door, she sees a package on her doorstep. Inside, she and her husband find a wooden box with a button protected by a glass dome, locked with a key, and a note, reading Mr. Steward will come at 5:00 pm.



Arthur goes to work at NASA, where he works in optics, and helped in designing the camera on the Viking Mars probe. He finds because he failed his psych exam, he has been rejected from the astronaut program despite good test scores and glowing recommendations. Norma goes to her job as a teacher at an elite private school teaching literature, where they are discussing Sartre's vision of Hell. One of her students remarks on her limp, after which she shows her class her disfigured right foot, missing four toes. Later that day Norma returns home and confesses her fear of losing their home, due to lack of money. That is when Arlington Steward appears at their door. Norma looks at the clock and realizes it is 5:00 pm.



Arlington offers them one million dollars if they press the button sealed in the dome. The catch is that someone they do not know will die. Norma and Arthur contemplate whether they would be able to cope with someone's death on their hands. After much discussion as to whether they should press the button, and some tinkering with the box (Arthur finds nothing inside it), Norma suddenly jerks forward and hits it.



Arlington returns and presents Norma and Arthur with the million, without asking whether they pressed the button. He informs the couple that whoever receives the offer next, they will surely not know them, implying that if the next people press the button, one of them, or their son, may be at risk. Arthur storms after Arlington and attempts to return the million, but Arlington drives off.



A 911 call is shown, where someone has been shot. The police enter the house and find a woman shot through the heart, and a little girl locked in the bathroom upstairs.



Arthur and Norma then attend a wedding rehearsal dinner, where Arthur is instructed to select a present from a table. A student of Norma's, the one who prompted her to reveal her disfigurement, holds up two fingers to Arthur. Arthur then sees a box much like the one left on their doorstep, with the button in it, and chooses that one. He and Norma find a poor quality picture of Arlington inside.



Norma's sister tells Arthur that the student Arthur saw was the one who made fun of his wife's foot. Arthur meets and asks Norma's dad (who is a police officer) to run the license plate number of Arlington's car. Norma receives a phone call (she is informed by a waiter whose nose begins to bleed as he's leading her to the phone). It is Arlington, who scolds her for allowing her husband to make contact with the police (Arlington somehow knows Arthur has spoken to Norma's father). Arthur angrily approaches the student from Norma's class and yells at him for making fun of Norma. He then storms out of the party, with Norma following him. As they start their car, they see "No Exit" (the title of the Sartre play they went to see earlier) written into the frost on their windshield.



When they get home, Arthur takes their babysitter, Dana, home. Dana had earlier gone down into the basement with their son Walter, to see Arthur's Mars collection. As they are driving, Dana acts strangely, telling Arthur to "look into the light" to solve his problems. Her nose begins to bleed and she passes out. Arthur attempts to wake her up and finds her drivers license, which shows her name is not Dana, but Sara, and she is from Boston. He reaches the motel, where "Dana" has been staying and she wakes up with a start. She tells Arthur it's not safe for him there, and to look in the mirror because that's the only place with the answer. She then hurries away. In the motel, every door she passes opens and a startled looking person stares her down. She reaches her room, where she has a large map and pictures of Arthur, Norma, and Walter.