Agatha Christie’s reputation as The Queen of Crime was built by the large number of classic tropes that she introduced, or for which she provided the most famous example. Christie built these tropes into what is now considered classic mystery structure: a murder is committed, there are multiple suspects who are all concealing secrets, and the detective gradually uncovers these secrets over the course of the story, discovering the most shocking twists towards the end. At the end, in a Christie hallmark, the detective usually gathers the surviving suspects into one room, explains the course of his or her deductive reasoning, and announces the guilty party.
Least Likely Suspect
Perhaps the most common element of the Agatha Christie plot is in fact the twist ending itself, that the suspect least likely to have committed the crime is guilty. The murderer tends to conceal either means, motive or opportunity, or a combination of all three. Often, if one of these is apparently watertight, the murderer will play up another: i.e., that he or she had clear access to the weapon, was desperate to murder the victim (as in Lord Edgware Dies), or was in the vicinity of the murder at the time of death (as in Five Little Pigs). This partial admission of guilt is considered an unlikely behavior of the murderer.
Detective Warns Murderer
Although there are many instances of characters warning each other of death, in Death on the Nile, Evil Under the Sun and the short story Triangle at Rhodes, the detective tells the murderer early on that "there is danger". It appears that the detective is warning them that they could be killed, but ultimately it turns out to have been incredible foresight on the part of the detective; the danger was of committing murder, being found guilty and hanged.
This is a very common clue to the reader that something specific should be regarded as relevant in the immediate events. In The Mystery of the Spanish Chest, a character seems to remember that there was something odd about a room. Poirot remembers that she is puzzled, and later prompts her to remember that a screen was in the wrong place. This is also used to heighten suspense as to whether the person concerned will "remember" the crucial item, or, having remembered it, whether they can communicate with the detective before being silenced, as for example in After the Funeral, where Helen Abernethie is attacked while trying to telephone through information about 'something odd' she has remembered which gives a clear clue as to the killer.
In Murder on the Links, Poirot draws the attention of Hastings to footprints in one of two flower beds. Hastings is misled into thinking that Poirot is interested in the footprints, but he is actually interested in their absence from the other bed, where they should have also been found.
This trope – which appears in several different forms throughout her novels – was borrowed by Christie from Arthur Conan Doyle’s short story, Silver Blaze. In this, Sherlock Holmes refers to “the curious incident of the dog in the night-time”, where the “curious” thing is the fact that the dog does not bark rather than that it does. Christie effectively admits the debt in the tenth chapter of Cards on the Table when her crime novelist character, Ariadne Oliver, explicitly mentions the source. The same reference is also specifically mentioned by Poirot in Murder in the Mews.
In the first chapter of Lord Edgware Dies, Hastings tells the reader that Poirot has always attributed his solution of this mystery to “a chance remark of a stranger in the street”. (The remark – “If they had just had the sense to ask Ellis right away” – has nothing directly to do with the mystery.) This is just one of many examples when the nature of the mystery is explained by an epiphany in which the detective makes a relevant discovery on the strength of a random occurrence.
Clues Hidden in Plain Sight
In Murder on the Links, Poirot stresses the potential importance of a length of lead pipe that is completely overlooked by a rival detective who only focuses on very small clues.
In a sense, many of Christie’s novels employ the same trope on a different level, in the sense that the murderer is rarely “the person one would least suspect”: more usually he or she is a character that has been very visible from early in the novel.
Supposedly Unreliable Character
In A Murder is Announced the silly and forgetful Dora Bunner tells Inspector Craddock what one particular character was doing shortly before the murder took place. But because she is so unreliable, everybody believes she was mistaken until she started to believe the version of the murderer herself. In The Mousetrap, Mrs Boyle points out that one character cannot be who he pretends to be, but nobody pays attention since Mrs Boyle is presented as a rather unpleasant woman who complains about everyone. In Crooked House, Brenda Leonides tells the narrator pretty early in the book that she thinks the character, who later turns out to be the murderer, might not be quite right in the head, but nobody believes her since Brenda herself is the main suspect in the poisoning of her much older and rich husband. In Dead Man's Folly one of the victims is an old man who found a dead body but his story was never believed.
Tampering with Time of Death
In several stories, the criminal plays with time, to make it look as though the crime took place when the criminal was elsewhere. In Evil Under the Sun the criminals fake a murder for a time when they both have alibis, then commit it later while the preliminary investigative bustle distracts attention. In Hercule Poirot's Christmas the murder is committed an hour before it appears to have taken place, at a time when the criminal is elsewhere in front of witnesses. In Hickory Dickory Dock, a criminal's accomplice makes a phone call that is ostensibly from the victim, at a time when the criminal is standing in front of Poirot. In The Plymouth Express and The Mystery of the Blue Train (the former being shortened version of the latter) the criminal disguises herself as the victim at a train station to create the impression that the victim was still alive when, in fact, she had been killed earlier. Playing with time invariably involves devices such as fake phone calls, gunshots, screams, disguises, people pretending to be dead, and other devices that take advantage of an observer's assumptions.
Murderer Plays Victim
In Peril at End House, a young woman (Nick Buckley) appears to be the target for a number of murder attempts. In fact she has arranged these in order to mask her own murder (of a distant cousin, Maggie) as another botched murder attempt that has miscarried. The same device for masking a real murder was used in A Murder is Announced and The Mirror Crack'd. Staged unsuccessful murder attempts appear in After the Funeral, Crooked House and Third Girl.
Viewpoint of the Murderer
In The Murder of Roger Ackroyd, the murder has been committed by the narrator, Dr. Sheppard, who never lies but omits mention of any of the actions that would demonstrate his guilt. The same thing happened in Endless Night.
While it is a common red herring to include unrelated minor crimes like robberies in the stories, in Cat Among the Pigeons two murders actually have no connection at all except for place and method. The second murderer just happened to mimic the first murder in execution.
False Corpses and Subsequent Murder
In Evil Under the Sun, the body of the victim is apparently discovered by two characters, one of whom goes to fetch the police. The murderer, however, has only “discovered” the body of his accomplice, and is left free to murder the real victim with a seemingly perfect alibi established. In Cards on the Table, the murderer finds his victim sleeping, tells the maid she's dead then kills her during the resulting confusion.
Premeditation and Misleading Clues
In Murder on the Links, most of the confusing elements of the crime are discovered to have been part of an elaborate plan by the victim to stage his own death and disappear. It is when he is happened upon by the real murderer that the final elements are added to the puzzle.
Similarly, in The Mystery of the Spanish Chest the victim himself plans to hide in the chest and catch his wife with the man that he suspects of being her lover. The murderer kills him while he is in the chest, resulting in a more complex situation to be solved than might otherwise have arisen.
In The Hollow, Poirot arrives at the scene of a murder in time to see a woman with a gun in her hand standing over the body of her husband, who is bleeding to death from a fresh bullet wound. It turns out at the end of the novel that she did in fact shoot him, but that this fact has subsequently been obfuscated by the other witnesses, all incriminating themselves to exonerate the woman for her perhaps justifiable act. Conversely, in Hickory Dickory Dock, the murderer is incriminated by so many clues that it appears he is being framed, with a lack of obvious motive and a clever false alibi for one of the murders weighing in his favor until the end. In Lord Edgware Dies, the murderer announces how she would kill the victim, and when doing it announces herself at the door of the victim's house perfectly truthfully, but has arranged apparent alibis to make it seem that she was framed. In The Murder at the Vicarage, the murderers each confess separately, but are cleared and only much later proved to be in fact guilty. In Towards Zero, the murderer fakes two sets of evidence that implicate himself in the murder of someone for which he has a blatant motive, only to plant a third set of evidence that frames someone else (and who in fact is the intended victim).
A variation on this is in Ordeal by Innocence, where the man found guilty for the crime, whose posthumously revealed alibi prompts a reopening of the case, turns out to have arranged the murder after all, though not committed it by his own hand.
Murderer Calls on Detective
In a number of stories, the criminal deliberately gets Poirot involved in the case. Of course it is only at the end that we discover this, and along the way it makes the real murderer less of a suspect to the reader. In Lord Edgware Dies, the murderer asks Poirot to help obtain a divorce, intending that this will help prove that she has no motive. In The A.B.C. Murders, the murderer sends letters to Poirot announcing the crimes beforehand, intending to frame an innocent person for the crime. A variation is Peril at End House, in which the murderer did involve Poirot deliberately, but until the end the reader is led to think her involvement is accidental.
Falsification and Murder Prevention
In some stories, such as After the Funeral and Murder In The Mews, a suicide or accident proves to be exactly that, but someone suggests otherwise, or comes along later and rearranges the scene in order to incriminate someone else. In And Then There Were None, the scheming murderer fakes his own being murdered, commits further murders, then commits suicide at the end so that it appears he was murdered much earlier and couldn't have committed any remaining murders.
In a few stories, such as The Labours of Hercules, someone who is thinking about committing murder is prevented from going any further. Interestingly, in the short story The Wasp's Nest, a variation on this theme is that the murderer has a terminal illness and plans to commit suicide in order to frame an intended victim, but is prevented from doing so.