While driving home from his London office in his Rover P5B, Harold Pelham, a director of Freeman, Pelham & Dawson, a marine technology company and very conservative creature of habit, seems to undergo a sudden personality change and starts to drive both fast and recklessly on his way home, imagining himself in a sports car, and ending in a serious high-speed crash. On the operating table he briefly suffers clinical death, after which there briefly appear to be two heartbeats on the monitor.
After he recovers from the accident Pelham notices odd things occurring and people acting strangely, and he gradually finds his life in turmoil. Friends, colleagues and acquaintances claim to have seen him in places where he has no memory of being or doing things he can’t recall, involving behaving in rash ways quite unlike his usual character. When he gets home from work, a friend is at his house for a drink which he doesn’t recall arranging, and an attractive girl at the company swimming pool casts him a knowing glance. At bedtime he and his wife have a somewhat tense but amicable discussion about their recent lack of a love life. His wife also notices a mysterious silver car (a Lamborghini Islero) which she sees parked outside their house, but gives it no further thought. The driver of the car is then seen lighting a cigarette and snapping the match stick in half after he blows it out, exactly as Pelham does.
There seems to be a spy at work trying to force a merger with a rival company. Pelham drives to the research and development centre in Rugby to try and see where the leak began.
Soon he suspects there is a “double” masquerading as him. On a night out at the company club with his wife, he hopes to energise their relationship by indulging her request to go gambling, but he is tense and clearly not interested. As they are about to leave he bumps into the attractive girl, who sees his wife a short distance away and says “I didn’t know you were married.” His wife notices the exchange and is furious, suspecting the worst. She threatens to leave him. He finds out where the girl lives and confronts her; confused, she makes it clear that “he” was having an affair with her. He angrily denies the affair; the woman, hurt and almost hysterical, yells at him to leave.
At his usual the barber‘s he is told, “I cut your hair yesterday”.
At work Pelham finds out that apparently he was supporting a merger that he now opposes with the board. He confronts an executive of the other company, who explains how the two of them had clandestinely arranged the deal in a series of meetings, to “his” (the double’s) benefit as well as the company’s, when “he” revealed a “top secret” technology breakthrough his company was about to make. When he confronts the rival firm (run by Ashton) he is reminded of three secret meetings: at the top of The Monument; in the London Planetarium; and in a boat on The Serpentine.
He phones home and due to a misunderstanding, his butler Luigi thinks he is asking for Mr Pelham. Luigi says “I will just get him”. He drives home quickly.
Distraught and unable to explain the unfolding events, he consults a psychiatrist, Dr Harris, and undergoes extended treatment in his clinic, where Harris explains that he doesn’t believe Pelham is mad but perhaps was acting out of a subconscious desire to break out of his obsessively rigid lifestyle. He agrees to be admitted to the psychiatrist’s clinic for a few days’ observation. On his discharge the doctor persuades him to adopt some less conventional behaviour, so he goes to work dressed quite differently. However, during his time away, the double finalised the merger and took his wife out on the town, culminating in their going home and sleeping together. Pelham calls his home from the office and is astonished when the phone is answered by someone claiming to be himself. On edge, he drives to his house as quickly as possible, and inside comes face to face with his double, who calmly insists he is the real Pelham, pointing out the uncharacteristic clothes the visitor is wearing. The family and his best friend are all there and side with the double.
After asking the others to let the two of them speak alone, the double tells the “real” Pelham that the new clothes were a mistake, and explains how on the operating table the double was “let out” but there is only room in this world for one of them. Both insist they will go to the police.
The real Pelham drives off in his Rover in a greatly agitated state. The double immediately leaves and pursues him in the sports car. Dr Harris happens to see both men and is shocked. After a high-speed chase in the rain, the two cars race towards each other on a bridge. The real Pelham swerves off into the river, and just before he hits the water his image fades away. The double stops and looks down into the water, and then, to the audible sound of a double heartbeat, he briefly clutches his chest as if in extreme pain, but the spasm soon passes and he becomes calm: there is only one Pelham again.