Andy Shaw is a divorced high school history teacher who spends his nights reading about the civilizations of Mesopotamia. One day Andy rents a movie recommended by a co-worker and sees an actor who looks exactly like him. Andy becomes obsessed with meeting the actor and spends weeks finding the actor's name. Posing as a film student and using his girlfriend's address, he sends a letter to the production company asking to get in touch with the actor. His relationship with his girlfriend, Patricia Heaton, deteriorates as he refuses to reveal his intentions. After obtaining the actor's phone number and address, Andy pursues his double, Carlos Walker, and eventually telephones him. Carlos's wife answers and mistakes Andy's voice for her husband's. Initially, Carlos dismisses Andy and refuses to meet, but later approaches him and agrees. They decide to meet in a week at Carlos' country house.
Andi buys a fake beard and visits Carlos. Upon arrival, the men take off their clothes and find that they are in fact physically identical. They have the same date of birth. They have the same voice and share the same scar and mole. Carlos asks Andy to clarify one more thing: He was born at the exact same time. He wants to know which of them is the "original" and which is the copy. Andy says that he was born at two in the afternoon. Carlos secretly informs Andy that he was born half an hour earlier, making him the original. Andy gets up to leave, saying that he at least has the compensation of knowing that Carlos will be the first to die, and that he will become the original in return. To this Carlos replied, "Well, I hope you enjoy thirty-one minutes of personal, complete and exclusive recognition, because from now on that's all you'll enjoy." The men agree that they have no reason to meet again, and Andy leaves.
Andy sends the fake beard to Carlos, who can't stop thinking about their encounter. Meanwhile, Andy and Patricia become engaged. Carlos wonders how Tertuliano got his phone number and address. He visits the production company's office and retrieves a letter written and sent by Andy in Patricia's name. Sporting a fake beard, Carlos sneaks out of Patricia's apartment and, finding her very attractive, he follows her to work. Carlos learns that Andy has not told Patricia about his double.
Soon after, Carlos comes to visit Andy at home. He shows Andy the letter with Patricia's signature. Threatened, Andy tells her to leave, saying he will call the police. Carlos says he will call Maria and tell her about Andy's forgery. Andy asks what he wants, and Carlos says he wants to spend the night with Patricia. Carlos, pretending to be Andy, has already contacted her and invited her to see his country house, his country house. Carlos seeks revenge for Andy's intrusion into his stable married life. Angry and embarrassed, Andy gives Carlos his clothes, identification, and his car keys.
After Carlos leaves, Andy exchanges his clothes for some in Andy's closet. He drives to Carlos' house in Carlos' car, where he makes love to Carlos's wife, Helena, that night. In the morning, she cooks him breakfast while he reads the newspaper without ever suspecting that he is not her husband.
Meanwhile, Patricia and Carlos spend the night together. In the morning, Patricia is the first to wake up and notices an indentation on Carlos's finger from her wedding ring. She guesses that he is not Andy and demands that she be let go.
Andy expected Carlos to return to find him in bed with Helena. As the time passes for Carlos to return home, he becomes concerned about Maria, leaves Carlos and Helena's house, and goes to a pay phone to call Patricia's house. A colleague of Patricia's answers the phone and tells him that Patricia died in a car accident that morning.
Andy checks into a hotel and calls his mother to tell her that he is alive. She meets him at the hotel and he tells her the whole story. The next day, he buys a newspaper to learn the details of the accident: a head-on collision with a truck. When questioned by the police, the truck driver said that it appeared that the passengers in the car were quarreling before crossing the middle lane and hitting the truck.
Andy returns to Carlos' house and reveals his identity to Helena, explaining that the one who died was her husband. He reveals Carlos's identity to her and apologizes to her. She replies: "Sorry is just a word." Helena asks Andy to stay with her and take her husband's place, and he accepts her invitation.
Three days later, while Andy is reading about the Mesopotamian civilization, the phone rings. He answers and a man on the other end of the line says, "At last!" In your own voice. The man says he has been trying to reach her for months and claims to be her double. Andy agrees to meet her at a nearby park that night. Andy changes into his clothes, loads the pistol he keeps in his house, and puts it in his belt. He writes a note to Helena, "I'll be back", and leaves for their rendezvous in the park.