Van Dine 4. The detective himself, or one of the official investigators, should never turn out to be the criminal.
Knox 7. The detective must not himself commit the crime.
From the very beginning of the detective novel the unmasking of the official investigator was considered a valid ploy. The Mystery of the Yellow Room (1907) by Gaston Leroux, creator of The Phantom of the Opera, is credited by Agatha Christie herself as being one of the two detective novels that she had actually read before embarking on The Mysterious Affair at Styles and contains one of the earliest examples of the criminal investigator. In The Clocks, Poirot, talking about his magnum opus on detective fiction, is unstinting in his praise for this groundbreaking novel. Some of Christie’s most deftly plotted books featured this ploy. Hercule Poirot’s Christmas was chosen by Robert Barnard in his Agatha Christie: A Talent to Deceive (1980) as one of the three best novels of Dame Agatha’s career, and indeed it is a classic English detective story of the type considered synonymous with the Christie school of whodunit, in other words a snowbound country mansion with a group of suspects and among them a killer. While her intentions when originally plotting this novel were completely different from those realised in the book we now know (see Agatha Christie’s Secret Notebooks), the solution is breathtaking in its daring and simplicity. We are given numerous clues to the true identity of Simeon Lee’s killer – the good looks, the habit of stroking the jaw, the subterfuge with the piece of rubber, the insistence on the family ‘on the other side of the blanket’, the daring exchange with Pilar in the chapter ‘December 24th’. But, like the presence of a narrator, Superintendent Sugden is not really seen by the reader, just accepted. With his unmasking, an ingenious (if somewhat unlikely) plot is revealed. An early foreshadowing of this ploy can also be found in ‘The Man in the Mist’ in Partners in Crime.
The Mousetrap, in both its stage and novella versions, and its earlier incarnation as the radio play Three Blind Mice, all unmask the investigator as the villain. Sergeant Trotter arrives like a deus ex machina in Monkswell Manor and is accepted unquestioningly both by its snowbound inhabitants and by the audience. In fairness, it should be said that although we think he is a policeman, he is actually an imposter, although the overall effect is the same. In the late 1940s and early 1950s the policeman, like the village doctor, was perceived as uncorrupted and incorruptible. Nowadays, unfortunately, we know differently and modern audiences are more likely to spot this type of villain than their more innocent counterparts of an earlier age.
In Curtain: Poirot’s Last Case, Agatha Christie played her last and greatest trick of all on her readers; and they loved her all the more for it. This is the ultimate sleight of hand from the supreme prestidigitator in the crime-writing pantheon. Who but Agatha Christie would have thought of, and then carried out, this almost sacrilegious trick? After 55 years of partnership, she unmasks Poirot as the killer. Certainly the book is contrived (which detective story is not?), but only the most churlish of readers would complain after such a dazzling culmination of two careers.
Van Dine 6. The detective novel must have a detective in it.
This is a perfectly reasonable Rule. But Agatha Christie made a career out of breaking the Rules, reasonable or otherwise, and she managed to demolish this one also. The most famous and best-selling crime novel of all time, And Then There Were None, has no detective. An epilogue is set at Scotland Yard where Inspector Maine and Sir Thomas Legge, the Assistant Commissioner, discuss the mass slaughter on the island but can offer no explanation that covers all the facts. It is left to a confession (breaking yet another Rule) to pinpoint the guilty party. Death Comes as the End is another example of a detective novel with no detective. Set as it is in Ancient Egypt 4,000 years ago, the absence of a detective is not remarkable. Clues also are necessarily in short supply; the fingerprints, cigarette ash and telephone alibis beloved of writers and readers alike are notable only by their absence.
Van Dine 9. There must be but one detective.
In the sense that Poirot and Miss Marple never meet between the covers of any of her books Agatha Christie abided by this Rule. But in many novels they work in close collaboration with the official investigators. And in other titles there is an unofficial coming-together of, effectively, suspects in order to solve the crime. In Three Act Tragedy, Death in the Clouds and The A.B.C. Murders Poirot agrees to co-operate with some of those under suspicion in order to arrive at the truth. And in all three cases one of his group of collaborators is unmasked in the last chapter. Coincidentally or otherwise, these novels were all published in the same 12-month period between January 1935 and January 1936.
The murderer
The other important figure, the murderer, also exercised both rule-makers. But Christie had broken most of these Rules before either Knox or Van Dine sat down to compose them.
Knox 1. The criminal must be someone mentioned in the early part of the story, but must not be anyone whose thoughts the reader has been allowed to follow.
While adhering to the former part of this injunction, the circumvention of the latter became almost a motif throughout Agatha Christie’s writing life. As early as 1924 with The Man in the Brown Suit she neatly and unobtrusively breaks this rule. Throughout the book we are presented with passages from Sir Eustace Pedler’s diary in which he shares his thoughts with the reader, before his eventual unmasking as the villain of the piece. The most famous, or infamous, example is, of course, The Murder of Roger Ackroyd. This title, her first for the publisher Collins, caused a major stir on its first appearance with its revelation of the narrator as a cold-blooded killer and blackmailer. The book immediately ensured her fame and success and it is safe to assert that, even if she had never written another word, her name would still be remembered today in recognition of this stunning conjuring trick. Forty years later she replayed it but in such a different guise that most of her readers were not aware of the repetition. While a doctor in a small 1920s village narrates The Murder of Roger Ackroyd, a young, working-class, charming ne’er-do-well narrates Endless Night. But it is essentially the same sleight of hand at work. (See also ‘Fairness’ above.)
More subtly, we share the thoughts of a group of characters, which includes the killer, in And Then There Were None, but without identifying which thoughts belong to which character (Chapter 11). And in The A.B.C. Murders we think we are sharing the thoughts of a serial killer when, in fact, he is the innocent dupe of the real killer. Less overtly, we are given an insight into the minds of the killer in Five Little Pigs, Towards Zero and Sparkling Cyanide.
Van Dine 10. The culprit must turn out to be a person who has played a more or less prominent part in the story.
Never one to cheat her readers, this is one of the Rules that Christie did not break, or not in the way that Van Dine intended. She never unmasked the second cousin of the under-housemaid as the killer in the last chapter. But adhering to the hidden-in-plain-sight ploy, the more prominent a part a character played the more suspicious should the reader be.
Van Dine 11. A servant must not be chosen as the culprit.
This is not mere social prejudice (although there is plenty of that in the work of Van Dine himself) but a practical solution to the problem of the unmasking, in the last chapter, of a member of the domestic staff whose presence in the novel was fleeting at best. Consider how Christie overcame this stricture. Kirsten Lindstrom in Ordeal by Innocence is, strictly speaking, a domestic servant but her significance to the Argyle family can be interpreted as placing her outside this category. But it is as a servant that we meet, and continue to perceive, her. This same consideration applies to Miss Gilchrist in After the Funeral; witness the telling scene at the denouement when she bitterly recriminates the Abernethie family. Gladys, in A Pocket Full of Rye, is a clearer example of domestic servitude. Indeed, it is her status as such that makes her a necessary part of Lance’s murderous plan. It is her job to poison the breakfast marmalade while Lance is demonstrably miles away, thereby giving him an impeccable alibi. But it is also a fact that, in defence