The book was heavily promoted upon publication in 1950 as being Christie's fiftieth book, although in truth this figure could only be arrived at by counting in both UK and US short story collections together to reach that total.
A strange notice appears in the morning paper of a perfectly ordinary English village, Chipping Cleghorn: "A murder is announced and will take place on Friday, October 29th, at Little Paddocks, at 6:30 p.m. Friends accept this, the only intimation." This comes as a great surprise to Letitia Blacklock, the owner of Little Paddocks, as she has no idea what the notice means; she didn't place it and none of her companions knows more than she. Miss Blacklock decides to take it in her stride and prepares herself to have guests that evening.
Naturally, the villagers are intrigued by this notice, and several of them appear on the doorstep with awkward reasons but a definite interest. As the clock strikes 6:30, the lights go out and a door swings open, revealing a man with a torch (flashlight).
In a heavily accented voice, the man demands they "Stick 'em up!" Most of the guests do so, believing it to be part of a game. The game ends when shots are fired into the room. The door slams shut, and panic takes hold: in short order, it's discovered that the fuses are blown, the gunman has been shot, and Ms. Blacklock is bleeding from a bullet's near-miss. The most curious thing of all is the gunman: Bunny recognizes him as Rudi Schertz, the receptionist at a local spa, who had asked Letitia for money just a few short days ago.
The police are called in. All clues suggest that the case is merely a strange suicide or accidental death, but Inspector Craddock is uneasy about both possibilities. As luck would have it, Miss Marple is a guest at the very same spa where Rudi Schertz was employed. Craddock is advised to involve her in the case, and the two commence working together. At the spa, it emerges that Rudi has a criminal background, but petty theft and forgery rather than any more serious crime. His girlfriend, however, reveals that not only had he been paid to appear, he believed it was all "a silly English joke": clearly he was not planning on being shot at.
With this new knowledge, Craddock returns to Chipping Cleghorn. Miss Marple, not uncoincidentally, is the godmother of the local vicar's wife, and decides to stay with her.
The first step is to establish a motive for Schertz's attack on Miss Blacklock. This presents a problem: Letitia has no known enemies. She worked for a successful financier (Randall Goedler) and has done quite well for herself but is not herself wealthy. She does not lead a lavish life as aside from her house, she has only enough to live on. However, she may shortly come into a great deal of money; Randall Goedler's estate passed to his wife, Belle, when he died. Belle is frail, and is now very near death. When Belle dies, Miss Blacklock inherits everything. If, however, she predeceases Bell, the estate goes to the mysterious "Pip" and "Emma", children of Randall's estranged sister. No one knows where these two are, much less what they look like.
Inspector Craddock discovers oil on the hinges of a door into the parlor (where the shooting took place) thought to be unused, and Bunny mentions that until quite recently there had been a table placed against the door.
Inspector Craddock travels to Scotland to meet Belle; she mentions that Letitia had a beloved sister, Charlotte, who was born with a goitre. Their father, an old-fashioned doctor, tried unsuccessfully to treat Charlotte, but she only withdrew further into herself as her goitre got worse. Their father died shortly before World War I, and Letitia gave up her job with Goedler and took her sister to Switzerland for the necessary surgery to repair the defect. The two sisters waited out the war in the Swiss countryside, but before it was over, Charlotte died very suddenly. Letitia returned to England shortly thereafter.
Miss Marple takes tea with Bunny, and Bunny reveals several details about the case: she talks about the recently oiled door she found with the Inspector; she's sure that Patrick Simmons is not as he appears; and, most tellingly, she's absolutely positive there was another lamp in the room on the night of the murder (the one with the shepherdess and not with the shepherd that is now in the room). Their tête-à-tête is interrupted, however, as Letitia arrives, and she and Bunny resume their shopping.
That evening, Letitia arranges a birthday party for Bunny, complete with all Bunny's friends and even a chocolate cake; this was while rationing was still in effect in England--butter and eggs were hard to come by even in a rural community, and chocolate was quite rare. Afterwards, Bunny complains of a headache and goes to bed after taking some of Letitia's aspirin as her own bottle of aspirin bought that morning seems to be missing. Bunny dies from poisoning in her sleep.
Miss Marple visits Ms. Blacklock who mourns Bunny and starts crying. Miss Marple asks to see photo albums which might contain pictures of Sonia Goedler, Pip and Emma's mother, but all photos of Sonia were taken out of the albums recently, although they were in place before the death of Rudi Shertz.
Through deduction and reenactment, Misses Hinchchliffe and Murgatroyd figure out that Murgatroyd could see who was in the room as she was standing behind the door when it swung open; she couldn't have seen Rudi as he was on the other side of the opened door, but she could see whose faces were illuminated by the flashlight beam. The two women conclude that the person who wasn't in the room (and therefore not seen by Miss Murgatroyd) could have snuck out of the room when the lights went out and come around behind Rudi, and shot at him--and Miss Blacklock!
Just as she remembers the one person not in the room, the stationmaster calls to notify them that a dog has just arrived. As Miss Hinchcliffe pulls away, Miss Murgatroyd runs into the driveway, shouting "She wasn't there!" She is murdered while Miss Hinchcliffe is away, and so does not reveal whom she did not see.
At Little Paddocks, Letitia receives a letter from the real Julia Simmons in Perth. She confronts "Julia" with the letter, and "Julia" reveals that she is actually Sonia's daughter, Emma Stamfordis, masquerading as Julia so that she and Patrick could be together and Julia could spend time acting.
Julia/Emma insists she is uninvolved in the assassination attempt--she was a crack shot during the French Resistance and would not have missed at that range, even in the dark--nor did she wish to disinherit Letitia. She had intended to identify herself to Letitia and ask for a small portion of the money, but once the murder took place, she thought it more sensible to continue the masquerade.
Philippa Haymes sneaks into the kitchen to speak to Julia, but Julia sends her away before finding out what Philippa had to say. That night, the vicar's cat, Tiglath Pileser, knocks over a glass of water onto a frayed electrical cord, which causes the fuses to blow, and the final clue falls into place for Miss Marple.
Inspector Craddock gathers everyone at Little Paddocks and launches the final inquest, which is interrupted by Mitzi, crying out that she saw Letitia commit the murder. The others do not believe her, as they recall seeing Letitia in the room.
Ms. Blacklock excuses herself, as she claims to feel ill. The inspector continues, and quickly discovers that "Julia" is Emma, but insists that Patrick is "Pip". Philippa says she is Pip. The Inspector accuses Edmund of being after Philippa's money, as he knew her true identity. Edmund denies this and as he does so, a terrified scream is heard from the kitchen.
Everyone rushes to the kitchen and discovers Miss Blacklock attempting to drown Mitzi in the sink, but is arrested by a local constable brought there by Miss Marple, in time to save Mitzi.
Miss Marple explains it quite simply: it wasn't Charlotte who died in Switzerland, but Letitia. Charlotte posed as Letitia and returned to England; few people knew Charlotte, as she had been a recluse before leaving England and a slight change in Letitia's appearance could be explained away to casual acquaintances by her time abroad during the war. She need only avoid people who knew Letitia well, such as Belle Goedler. Bunny was one of the few people who remembered Charlotte as Charlotte, but by then, Charlotte was so lonely that she allowed her old school friend to move in.
However, Rudi Schertz could have ruined everything: he worked at the Swiss hotel where the sisters had stayed and could therefor also identify Charlotte as herself. This is why "Letitia" hired him to come to Chipping Cleghorn and "hold up" a room full of guests: she blew the fuse by pouring water from a vase of flowers onto the frayed cord of a lamp, slipped out the second door, stood behind Rudi, and shot him. She then nicked her ear with a pair of nail scissors and rejoined the others, playing the part of perplexed host.
Bunny became the next target because she, too, could reveal too much. Bunny had an eye for detail, but was prone to slip-ups: on several occasions, she referred to Ms. Blacklock as "Lotty" (short for "Charlotte") instead of "Letty" (short for "Letitia"), and her conversation with Miss Marple in the cafe proved fatal.
Miss Murgatroyd, the final victim, was also killed for guessing too much.
Mitzi and Edmund had been persuaded by Miss Marple to play parts in tripping Charlotte Blacklock up; Miss Marple's plans were almost brought down when Philippa admitted to being Pip, but Inspector Craddock thought fast enough to turn around and claim Edmund was after Philippa's money.
After five years of not reviewing any Christie detective novel, Julian MacLaren-Ross in the Times Literary Supplement was fulsome in his praise of the book in the issue dated June 23, 1950: "A new novel by Mrs Agatha Christie always deserves to be placed at the head of any list of detective fiction and her fiftieth book, A Murder is Announced, establishes firmly her claim to the throne of detection. The plot is as ingenious as ever, the writing more careful, the dialogue both wise and witty; while suspense is engendered from the very start, and maintained skilfully until the final revelation: it will be a clever reader indeed who anticipates this, and though Miss Christie is as usual scrupulously fair in scattering her clues, close attention to the text is necessary if a correct solution of the mystery is to be arrived at before the astute Miss Marple unmasks the culprit."
The review concluded, "Miss Christie has several surprises up her sleeve besides the main one, and (this much may be said without spoiling the reader’s pleasure) she once again breaks new ground by creating a weak and kindly murderer who is yet responsible for the deaths of three people: that such a character should, in the last analysis, seem credible, is a tribute to the author’s psychological acumen and originality of concept.
Maurice Richardson, in the June 4, 1950 issue of The Observer said, "For her fiftieth book she has chosen a snug, residential village setting with her favourite detective, silver-haired, needle-sharp spinster, Miss Marple, making a delayed appearance. Not quite one of her top notchers, but very smooth entertainment. The Prime Minister (Clement Attlee), who is her fervent admirer, might fittingly celebrate this jubilee by making her a Dame. (In the event, it took until 1971 for Christie to be awarded the DBE).
Normal Shrapnel in The Guardian's issue of June 9, 1950 noted that this was Christie's fiftieth book and said that the murderer was, "run to earth in a brilliantly conducted parlour game".
An unnamed reviewer in the Toronto Daily Star of September 30, 1950 said, "A Murder is Announced displays all the adriot and well-bred legerdemain one has come to expect from Agatha Christie...This jubilee whodunit is as deft and ingenious a fabrication as Agatha Christie has contrived in many a year.
Robert Barnard: "Superb reworking of the standard Christie setting and procedures, marred only by an excess of homicide at the end. The book is distantly related to The Companion, in The Thirteen Problems.
The novel was serialised was in eleven parts in the Daily Express from Tuesday, February 28 to Saturday March 11, 1950. Five of the instalments carried an illustration by long-term Express artist Andrew Robb. This version did not contain any chapter divisions and contained only about half of the text that appeared in the book publication, totally omitting chapters five, six, seven, fourteen and the epilogue. It had been planned for this serialisation to take place closer to the eventual book publication in June 1950 but it was pulled forward by Christie’s literary agent Edmund Cork in an effort to boost interest at the ailing box office for the play Murder at the Vicarage.
In the US, the first publication was in the Chicago Tribune in forty-nine parts from Monday, April 17 to Monday, June 12, 1950.