The screenplay was written by Knott, who moved to the U.S. in 1954 and wrote only one other well-known play, Wait Until Dark (1966), which was filmed a year later. Knott's work tends to focus on women who innocently become the potential victims of sinister plots.
There is just one setting in the stage play of Dial M for Murder: the living-room of the Wendices' flat in London (61A Charrington Gardens, Maida Vale). Hitchcock's film adds a second setting in a restaurant, a few views of the street outside and a stylized courtroom montage. Having seen the play on Broadway, Cary Grant was keen to play the role of Tony Wendice, but studio chiefs did not feel the public would accept him as a man who arranges to have his wife murdered.
In June 2008, the American Film Institute revealed its "Ten Top Ten" — the best ten films in ten "classic" American film genres — after polling over 1,500 people from the creative community. Dial M for Murder was ranked the ninth best film in the mystery genre in the AFI's list.
Tony and Margot have made their wills, naming each other as beneficiary. For a year, Tony meticulously plans Margot's murder. She has no idea that Tony knows of her love for Mark. He has gone to great lengths to steal a handbag containing one of Mark's letters, and even assumed the role of an anonymous Brixton-based blackmailer to find out whether she would pay to have it back.(She did, but he asked for only £50.) He even watched them having a little farewell party (eating spaghetti with mushrooms) in Mark's studio flat in Chelsea.
Tony slyly withdraws small amounts of money for a year, collecting £1,000 in (used) one-pound notes, with which he plans to pay a contract killer. He singles out the perfect man to do the job: C. A. Swann (Anthony Dawson), who now calls himself "Captain Lesgate", a former acquantaince who has embarked on a life of petty crime since even before leaving Cambridge where he and Tony were both students. By following him and finding out about his past and associations, Tony soon gets enough to blackmail Swann into murdering his wife.
Tony uses the opportunity of Mark's return to London to carry out his plan. Under a pretext—he has to prepare an urgent report for his boss—he has Margot and Mark go to the theatre and, when they are gone, he invites Swann to his flat under another pretext—wanting to buy an expensive car from him. When Swann arrives at 61A Charrington Gardens that night, Tony gets down to business. There is no time to lose, as he has planned the murder for the following night. Trapped in a corner by the revelations of his past crimes and tempted by the money, Swann agrees.
Tony has invited Mark to join him at a stag party in a nearby hotel—this is how he secures himself an Alibi. The idea is that the police should think that a burglar was surprised by Margot, that he panicked, strangled her and left without the loot. He has told Swann that he is going to phone his own flat at exactly 11 p.m. so that Margot will come to the living-room to answer the phone, whereupon she will be murdered by Swann. There are only two keys to the Wendices' ground floor flat. Before leaving for the stag party, Tony steals Margot's key from her handbag and hides it under the stair carpet outside their flat for Swann to use.
Mark, a writer of crime scenarios, says at one point that, theoretically, he would be able to plan the perfect murder but that it would be impossible to carry out any plan of his because in real life people just do not act according to other people's plans. This is true of Margot, too: Instead of listening to the radio in her bedroom when Tony and Mark are away, she tells her husband of her own plans to go to the cinema that night. Tony has a hard time persuading his wife to instead stay at home and stick into an album some old newspaper clippings of his when he was a tennis star. Margot finally consents and for that reason takes a (seemingly) huge pair of scissors out of her mending basket (which also contains a pair of her stockings). When she has finished the tiresome job she goes to bed, carelessly leaving the scissors lying on the desk next to the phone. According to Tony's plan, Swann secretly enters the Wendices' flat shortly before 11 o'clock, hides behind the drawn curtains, a scarf in his hands, and waits for the telephone to ring and for Margot to come out of her bedroom to answer it. When she does, the plan goes terribly wrong: Swann attacks her from behind—with Tony all the while listening in to what is going on over the phone—but Margot turns out to be rather strong and eventually stabs Swann in the back with the scissors. He falls to the floor and is dead at once. In his panic, Tony tells his sobbing wife not to touch or do anything until he has come home, which he hurriedly does. After getting home, he calls the police.
Tony's mind has to work fast now if he wants to come up with an alternative plan. He realizes he can make it look as if Margot had been blackmailed by Swann, that the blackmailer came to her flat in person and that she actually let him in with the intention of murdering him (rather than killing him in self-defense). By now it has been established that Swann came in through the hall door rather than the French windows leading into the garden, as his shoes are not dirty. This would mean that she will be hanged, and that he will inherit her money after all.
In the course of the police investigations, led by Inspector Hubbard (John Williams), Tony succeeds in cunningly and artfully planting clues in a way that gets his unsuspecting wife deeper and deeper into trouble. For one thing, he hides Swann's scarf (in the film, he burns it in the fireplace), replaces it with one of Margot's stockings from her mending basket and hides the other stocking beneath the blotter on the desk. For another, before the police arrive at the scene of the crime, he puts Mark's letter into one of the inside pockets of the dead man's suit—which will go to show that he actually was blackmailing Margot. Also, he extracts Margot's key from one of Swann's pockets and puts it back into his wife's handbag. Soon Margot is seen as the main suspect; she is arrested, tried, and sentenced to death.
There are two things Tony has not reckoned with: (a) that Swann replaced the key under the stair carpet immediately after using it rather than when leaving the flat again and that, accordingly, the key Tony takes out of the dead man's pocket is the key to Swann's own flat; and (b) that getting rid of £1,000 in cash (the money he would have paid to Swann, which he does not have to now that he is dead) by making many purchases is a conspicuous thing to do, bound to be investigated by the police. They do, but Tony is not aware of it.
On the day before Margot's scheduled execution, Mark visits Tony to propose a very unusual thing to him. Rather than seeing his wife hanged, he could come up with a completely new story, confess at the last minute that he hired Swann to kill his wife and save her life by going to prison for some years himself instead. Coincidentally, Mark has come up with exactly what Tony actually did. Mark argues that during Margot's trial, all arguments revolved around three things only: (1) Mark's letter found on Swann; (2) the fact that no key was found on Swann (and that there was no forced entry either); and (3) Margot's stocking. Mark argues that all this could be altered, and that Tony could put all the blame on himself, claiming that it was he who had done all that.
Then Inspector Hubbard arrives at the flat again, allegedly to ask Tony about the money he has been spending lately. This is when Mark discovers Tony's attaché case filled with the remaining one pound notes. Pressed for an answer, Tony manages a final impromptu lie in front of both Mark and the police: He tells them this is the money Margot had ready when she met Swann but that she changed her mind and killed him instead of paying him off. The inspector accepts this explanation and dismisses Mark's theory, or so it seems.
In fact, the inspector, who has not given up the case yet, remains suspicious of Tony and sets a trap. He borrows the key from Margot's handbag — which is kept in prison — and goes to Tony's flat while Tony is not home. He finds that it does not fit the lock. He discovers that the key actually fits Swann's door. After some searching he discovers the actual key under the carpet.
Then the inspector uses his final trick. He visits Tony to ask some questions and when he leaves he deliberately takes Tony's raincoat instead of his own. (The raincoats are apparently very similar and so are the keys.) Tony also leaves his flat. Inspector Hubbard secretly enters the flat, using the key from the raincoat, telephones the prison and asks that Margot be released. Margot, who does not understand why she has been released, goes home, and finds that the key in her handbag does not fit the lock. Hubbard waits some time, he wants to find out whether she knows the hiding place under the stair carpet. She does not, so that clears her of any suspicion. The inspector opens the door for her. Hubbard tells a colleague to take the handbag back to the prison. Hubbard and Margot then wait for Tony's homecoming.
When Tony comes home he realizes that he cannot get inside: he is wearing Hubbard's raincoat with Hubbard's key. Hubbard and Margot hide themselves inside and do not open the door. Then Tony remembers that he had been told to collect Margot's belongings from the prison, so he goes there to fetch her handbag, with the key. When he comes home he finds that the key from Margot's handbag — actually Swann's — does not fit into the lock. Tony starts thinking what could have happened. When he takes the key from under the stair carpet he gives himself away.
Tony enters the room to find Margot and the inspector, and Mark too. He realizes he's been found out and congratulates the inspector. He then offers everyone a drink, acting very casual, as tears begin to stream down his wife's face. The last scene is of the inspector, acting in a manner that shows he's proud of himself, as he combs his mustache.
A commentary on Dial M for Murder ascribed to Hitchcock goes like this: "As you can see, the best way to do it is with scissors." This refers at the same time to the film's pivotal scene, in which Grace Kelly stabs her would-be murderer with a pair of scissors, and to the clever editing which is a hallmark of his movies. One of the finest scenes is when we see Tony Wendice at the stag party, slightly nervous and frequently looking at his watch. It is already past eleven when he notices that it has stopped: He gets up from the table, hurries to the phone booth, has to wait there and eventually calls his flat well after 11 o'clock, at the very moment Lesgate is about to leave it again, believing that he has waited in vain. This is a miniature race against time full of dramatic music, complete with a cut to the automatic telephone exchange.
There is no real courtroom scene. This part of the film is done in a highly stylized way: The camera is on Margot, there are no props (only various colored lights), and the various people present at a trial are only introduced by means of voice-overs. Margot being sentenced to death is altogether missing from the stage play; it is only reported.
Apart from a few short outdoor shots—Tony Wendice approaching and leaving his flat etc.—the claustrophobic atmosphere of other Hitchcock films (Rope, Rear Window) can also be found here. Most of the action is restricted to a single set. The angle of the camera is also of interest (several times shot from the ceiling, a sort of bird's eye view).
Alfred Hitchcock's cameo is a signature occurrence in most of his films. In Dial M for Murder he can be seen (13 minutes into the film) in a black-and-white reunion photograph sitting at a banquet table among former students and faculty.
A Perfect Murder is a 1998 remake directed by Andrew Davis and starring Michael Douglas and Gwyneth Paltrow in which the characters of Halliday and Lesgate are combined: the husband (Douglas) hires his wife's lover (played by Viggo Mortensen) to kill her. However, a twist occurs when the lover sends a third party to kill the wife. The part of the inspector (David Suchet) is also much reduced, and it is Gwyneth Paltrow's character (as the wife) who unravels much of the mystery.
The character played by Robert Cummings of TV crime writer Mark Halliday, was originally called "Max Halliday" in the stage play. In the 1956 US TV series Alfred Hitchcock Presents there is an episode called 'Portrait of Jocelyn' that features a man called Mark Halliday, who murders his wife.