The book features the characters of Hercule Poirot, Arthur Hastings and Chief Inspector Japp. The form of the novel is unusual, combining first-person and third-person narrative. Christie had previously experimented with this approach (famously pioneered by Charles Dickens in Bleak House), in her novel The Man in the Brown Suit. What is unusual in The A.B.C. Murders is that the third-person narrative is supposedly reconstructed by the first-person narrator, Hastings. This approach shows Christie's commitment to experimenting with point of view, famously exemplified by The Murder of Roger Ackroyd.
Poirot and the police are baffled until a series of clues lead them to suspect the murderer is traveling as a stocking salesman. Then the 'D' murder in Doncaster goes awry and a stocking salesman called Alexander Bonaparte Cust walks into a police station and surrenders.
The case seems closed, but although Cust has confessed to the crimes, he claims not to have heard of Hercule Poirot and can not explain the letters, although they were written on his typewriter. Cust suffers from epilepsy and is subject to blackouts. He claims he can not recall committing the murders, but he believes he committed them because he was in the vicinity of each crime scene. He also sees other clues, such as blood on his cuff and believes himself the culprit. Poirot is suspicious and is later able to prove that Cust is innocent of the crimes.
In a twist ending Poirot reveals that the brother of Sir Carmichael Clarke, Franklin Clarke, who wanted Sir Carmichael's property and money, committed the crimes in order to draw attention away from the murder of his brother. Franklin had met Cust by chance and decided to use him as part of his plan. He arranged for Cust to be hired as a stocking salesman and gave him a travel itinerary that ensured he was at the scene of each murder. It is also revealed that Clarke had put the blood on Cust's cuff. He also sent Cust a box of ABC Railway Guides and a typewriter, on which he had already typed the 'ABC' letters.
The story features various characters, associated with the victims, who need to be investigated for possible means and motive for the murders:
The Times Literary Supplement of January 11, 1936 concluded with a note of admiration for the plot that, "If Mrs. Christie ever deserts fiction for crime, she will be very dangerous: no one but Poirot will catch her.
Isaac Anderson in The New York Times Book Review of February 16, 1936 finished his review by stating, "This story is a baffler of the first water, written in Agatha Christie's best manner. It seems to us the very best things she has done, not even excepting Roger Ackroyd.
In The Observer's issue of January 5, 1936, "Torquemada" (Edward Powys Mathers) said, "Ingenuity...is a mild term for Mrs. Christie's gift. In The A.B.C. Murders, rightly chosen by the [crime] club as its book of the month, she has quite altered her method of attack upon the reader, and yet the truth behind this fantastic series of killings is as fairly elusive as any previous truth which Poirot has had to capture for us. The reader adopts two quite different mental attitudes as he reads. At first, and for a great many pages, he is asking himself: "Is Agatha Christie going to let me down? Does she think she can give us this kind of tale as a detective story and get away with it?" Then the conviction comes to him that he has been wronging the authoress, and that he alone is beginning to see through her artifice. In the last chapter he finds, because brilliant circus work with a troop of red horses and one dark herring has diverted his attention from a calm consideration of motive, he has not been wronging, but merely wrong. It is noticeable, by the way, that characters break off at intervals to tell us that we have to do with "a homicidal murderer." We are ready to take this for granted until Mrs. Christie (I wouldn't put it past her) gives us one who isn't.
E. R. Punshon reviewed the novel in the February 6, 1936 issue of The Guardian when he said, "Some readers are drawn to the detective novel by the sheer interest of watching and perhaps anticipating the logical development of a given theme, others take their pleasure in following the swift succession of events in an exciting story, and yet others find themselves chiefly interested in the psychological reactions caused by crime impinging upon the routine of ordinary life. Skilful and happy is that author who can weave into a unity this triple thread. In Mrs. Agatha Christie's new book…the task is attempted with success." He went on to say, "In the second chapter, Mrs. Christie shows us what seems to be the maniac himself. But the wise reader, remembering other tales of Mrs. Christie's, will murmur to himself 'I trust her not; odds on she is fooling me,' and so will continue to a climax it is not 'odds on' but a dead cert he will not have guessed. To an easy and attractive style and an adequate if not very profound sense of character Mrs. Christie adds an extreme and astonishing ingenuity, nor does it very greatly matter that it is quite impossible to accept the groundwork of her tale or to suppose that any stalking-horse would behave so invariably so exactly as required. As at Bexhill, a hitch would always occur. In the smooth and apparently effortless perfection with which she achieves her ends Mrs. Christie reminds one of Noel Coward; she might, indeed, in that respect be called the Noel Coward of the detective novel.
An unnamed reviewer in the Daily Mirror of January 16, 1936 said, "I'm thanking heaven I've got a name that begins with a letter near the end of the alphabet! That's just in case some imitative soul uses this book as a text book for some nice little series of murders." They summed up, "It's Agatha Christie at her best.
Robert Barnard: "A classic, still fresh story, beautifully worked out. It differs from the usual pattern in that we seem to be involved in a chase: the series of murders appears to be the work of a maniac. In fact the solution reasserts the classic pattern of a closed circle of suspects, with a logical, well-motivated murder plan. The English detective story cannot embrace the irrational, it seems. A total success – but thank God she didn't try taking it through to Z.
In chapter three of the novel, Poirot lays out the plot of what he considers a perfect crime, a crime so challenging that 'even he' would find it hard to solve. This exact murder - where someone is murdered by one of four people playing Bridge in the same room with him - is the subject of Christie's Cards on the Table, which was published later in the same year. Similarly, in chapter one of The A.B.C. Murders Poirot alludes to a situation in the 1935 novel, Three Act Tragedy.
The first adaptation of the novel was the 1965 film The Alphabet Murders with Tony Randall as Hercule Poirot.
The novel was adapted in 1992 for the television series Agatha Christie's Poirot with David Suchet playing the role of Hercule Poirot. The adaptation remains faithful to the novel, with some minor changes and characters omitted. The cast included:
The first true publication of The A.B.C. Murders occurred in the US with an abridged version appearing in the November 1935 (Volume XCIX, Number 5) issue of Cosmopolitan magazine with illustrations by Frederic Mizen. The UK serialisation was in sixteen parts in the Daily Express from Monday, November 28 to Thursday December 12, 1935. All of the instalments carried an illustration by Steven Spurrier. This version did not contain any chapter divisions and totally omitted the foreword as well as chapters twenty-six, thirty-two and thirty-five. In addition most of chapters seven and twenty were missing. Along with other abridgements throughout the novel, this serialisation omitted almost forty percent of the text that appeared in the published novel.