The film begins with a CIA internal memo being typed across the screen. It states that a man named Arlington Steward (Frank Langella) has recovered from severe burn wounds and is delivering units related to the Mars project.
The film then opens up in 1976 with Norma (Cameron Diaz) and Arthur Lewis (James Marsden) cuddling in bed 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. Norma and Arthur then enjoy a nice breakfast with their son, Walter (Sam Oz Stone).
Arthur goes to work at NASA, where he works in optics, and helped in designing the camera on the Viking Mars probe. He finds he has been rejected from the astronaut program due to the fact that he failed the psychological exam.
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. After class Norma is informed by her boss that she will no longer be getting a discount for tuition and won't be able to pay for school for her son. Later that day Norma returns home and Arlington Steward appears at their door. Half of his face is missing. Norma looks at the clock and realizes it is 5:00 pm.
Arlington says that if she or her husband pushes the button, someone in the world that they do not know will die, and they will receive one million dollars. Arlington hands Norma a $100 bill from the brief case of money, and says she gets to keep it if the button is pushed or not. Arlington leaves and tells her she has 24 hours. Arthur returns and Norma tells him what has happened.
After much discussion about whether or not they should press the button, and some tinkering with the box (Arthur finds nothing inside it), Norma suddenly jerks forward and hits it. The button rises slightly.
Arlington returns and presents Norma and Arthur with the million, without asking whether they pressed the button or not. He informs that they will not know who will receive the offer next, and implies that Arthur, Norma, or their son will be the ones to die. Arthur storms after Arlington and attempts to return the million, but Arlington drives off. Arthur gets his license plate number.
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 (secret Santa). A waiter (who happens to be the student of Norma's 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. This time, there a photograph 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, as 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. She notices the picture of everyone who worked on the development of the camera. As they are driving, Dana acts strangely, asking Arthur strange questions such as "Whats pushing your buttons?" and comments like "Look into the light." Her nose begins to bleed and she passes out. Arthur attempts to wake her up and finds her driver's 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.
At home Arthur looks at the large picture in the basement, and sees that Arlington (prior to his injury) is in the photo.
At a supermarket, Norma is approached by a panicked woman who tells her to look up a certain call number in the library, and not to trust her husband, before passing out with a bloody nose. Arthur finds out that Arlington's license plate is registered to the NSA. He asks Norma's father if he can go with him to see the house where the shooting mentioned earlier took place. Once there, he finds pictures of Arlington and a picture of a Human Resources book, and a library call number. Arthur learns that the woman was shot at 4:45 PM, the same time Norma pushed the button.
Norma and Arthur both visit the library, separately. Norma avoids Arthur, as instructed by the woman in the supermarket. Norma finds a film reel, which shows Arlington prior to his disfigurement. Arthur is followed by a crowd of startled looking people. He finds himself in a large hall filled with more scared looking people. He approaches a woman he learns is Arlington's wife, and she tells him to follow her. He is presented with three "gateways," made of hovering water. Two lead to eternal damnation, one to salvation. Remembering the student who held up two fingers, Arthur picks gate two. He enters the water and finds himself whirling through whiteness.
Norma is also led by two hollow looking women to Arlington. He asks her how she felt when she saw his disfigurement, and she says she felt love, because of her foot. She says that she felt bad for herself because of her foot, and seeing someone with a face disfigurement made her realize she would never feel sorry for herself ever again. Many years ago, her brother had dropped a barbell on her foot and when she went to the doctor he put it in an X-ray machine and forgot about her, destroying the tissue in four of her toes. Arlington informs Norma that he was struck by lightning, and can now communicate with "those who control the lightning." Norma begins to cry and Arlington takes her hand.
Norma is suddenly lying on her bed, with Arthur suspended above her in the same hovering water he stepped into earlier. Norma moves away just before Arthur falls out of the water. The water falls too, going all the way down their stairs. When cleaning up, Walter demands to know what is going on. Norma and Arthur do not answer him.
More drama occurs and eventually, at the wedding of Norma's sister, their son Walter is kidnapped. Arthur is taken away by a gun-toting former employee of NASA. He is the same man who shot his wife, as earlier seen in the 911 call. He reveals to Arthur that he had to choose between his wife or his daughter. He asks if it was him or Norma that pushed the button and he replies that his wife also pushed the button. He shows Arthur the book seen earlier in the picture, and the water portals or "triptychs." The book is some sort of manual. They are then stopped by a man in a Santa Claus uniform, ringing a bell. As the two men are trying to figure out what is going on, they are hit by a large truck.
We then see Arthur emerge from a NASA warehouse, which has been surrounded by military. He is taken away and told by his NASA friend that everything that happens next will have great ramifications. Arthur and Norma return home and see Arlington in their kitchen. He informs them they face two final options. Their son, Walter, is now deaf and blind. They can either live on with their million dollars, and their disabled son, or Arthur can shoot Norma through the heart, at which point Walter's sight and hearing will be restored and the million will be placed in a high interest bank account for Walter to be held in trust until his eighteenth birthday.
We learned earlier in a warehouse full of startled people, when Arlington's minion questioned him, that Arlington's employers are testing the human race to see if they are worth keeping.
Arlington leaves, telling Norma and Arthur on his way out that their son is in the bathroom, locked upstairs. The two of them run upstairs and try to get him out, but cannot because he can't hear them pounding on the door. They decide that they must make a choice, and Norma leads Arthur downstairs.
Another couple is sitting at a table with a box unit, staring at it with contempt, wondering whether they should push it. The wife pushes the button. Arthur and Norma are crying and hugging each other, and say goodbye.
Arthur shoots Norma, and the police arrive. He runs upstairs and sees his son is back to normal. It is implied that this process will go on and on.
Arlington arrives at the other couple's house and hands them the million.
Arthur's NASA friend informs him he and his son will be taken care of, as Arthur is escorted away by men in uniforms, who put him into a black car before driving off. Arthur's son looks down at Arthur leaving in that black car. Arlington then comes out of the house of other couple to whom he had given the $1 million, and the film ends.