I’ve recently watched the episodes in the thirteenth and final series of Agatha Christie’s Poirot, starring David Suchet. It included Curtain, which of course must be the last episode, but it had several episodes which differed very greatly from their source material. In particular, The Big Four and The Labours of Hercules.
The former was described by the screenwriter as an unadaptable mess, which it certainly seems to be looking at the plot summary. It is basically a spy thriller with dozens of characters set throughout Europe, which is not very viable for a TV show, even if it is nearly two hours long. The one which really interests me, though, is The Labours of Hercules. The original is a collection of twelve unrelated short stories, which the screenwriter turned into a single long-form mystery by taking one of the stories as the central one and using several of the other stories as the red herrings which one expects in a Christie novel. At this point, I should warn you that this post will include spoilers. You have been warned.
Given what a challenging prospect that is, the writer did a good job, but there were problems in the story which I do not think were avoidable for structural reasons. As everyone knows, a murder mystery must have suspects, with the plural being imperative. Every man having free will, anyone who was anywhere near the victim is a suspect, which is why an isolated setting—a mansion, a private island, etc.—is so interesting. Unless the author is cheating, the suspect list is known at the outset. When doing this, the author must be very careful to make all of the suspects believable suspects. That’s a universal criteria, but a murder in the middle of a city means that we see a great deal less of the suspects, so each one has far greater scope for unseen action, including accomplices we don’t know about yet, than people in an isolated setting.
The episode, The Labours of Hercules, was set in a hotel on the top of a mountain in Switzerland, with the funicular train that is their only link to the outside world having been shut down by an avalanche. Short of a ship in the middle of the Atlantic ocean or an aeroplane in the sky, it’s about as isolated as it is possible to get.
The central mystery, though Poirot stumbles onto it almost by accident, is the identity of a psychopathic killer and thief called Marrascaud. The mystery was set up in the beginning where Marrascaud managed to kill several people and steal several valuable items—one of them a large painting—from a crowded building, with disguised policemen and Poirot himself protecting them. From this we know that Marrascaud is a genius on a level with Poirot, and this forms the central problem once we get to the hotel.
As has been observed in countless murder mysteries, the best place to hide a leaf is in a forest; to hide a genius one must really put them amongst other geniuses, but the characters at the hotel were taken from other stories and thus had qualities appropriate to those stories, none of which involved unique genius. In this case, the beautiful daughter of Poirot’s former love interest who is fascinated with criminology stands out almost like a sore thumb; the only other person who comes close is the Countess Rossakoff, her mother, but it was very clearly established in the previous episode where we met the Countess that the character is not a murderer. Marrascaud kills for the pleasure of it, brutally, which is not something one degenerates to in old age. It is true that one can be cruel vicariously, through underlings, in old age, but it does not make sense as a personality change to go from an honorable thief to a psychopath who delights in killing.
An interest in criminology is also something of a red flag in a suspect. Though everything has by this time been used as a false flag in detective fiction, none the less the similarity of the violent nature of both crime and law enforcement is unavoidable. As the saying goes, the main thing which distinguishes a sheep dog from a wolf is who it bites. None of the other guests seemed sufficiently… canine.
I think that this is the reason why Conan Doyle put Moriarty as the mastermind, behind the scenes. The proxy of an evil genius need only be of ordinary intelligence, which makes it far easier for him to blend in. Indeed, executing a plan which requires greater intelligence than he himself possesses serves as a form of camouflage for the immediate villain. Still, as bumbling accomplices have long shown, it is best to choose someone intelligent enough to understand the plan once it has been created; an accomplice who can understand only his part and not what it fits into will make mistakes that will prove the undoing of both.
I think that fact is why some villains have tried to manipulate their accomplice into helping without realizing it; if done well the mistakes of the unwitting accomplice actually hide the involvement of the mastermind. I suspect that this is the ideal strategy for the criminal mastermind; it is the safest type of plan if a brilliant detective shows up. If done extremely skillfully, it is possible to conceal that there even is a brilliant plan at work; the brilliance can be disguised as coincidence.
Of course, mysteries can go the other way—the more realistic way—where the detective must make sense of genuine coincidences. The problem with writing this sort of mystery is that it is extremely difficult to pull off without the detective himself getting lucky. And while a comedic detective—Inspector Clouseau, for example, or taking the idea of a detective very loosely, Maxwell Smart—can stumble onto all of his solutions, it’s not entertaining if a serious detective does that. Though, I should mention that this is why Jessica Fletcher almost invariably figures out the solution of most episodes by chance. In order to make Murder, She Wrote accessible to a general audience, the writers would tend to throw in enough clues that one should be able to figure out the solution before Jessica does. Since Jessica does have to figure out the mystery, something must make her realize the solution, and because we the audience are supposed to already get it, it can’t be the last piece of critical evidence, but nor can it be slam-dunk evidence, because then you couldn’t feel smart during the reveal. So it’s usually something silly somebody says, and then Jessica says, “Wait, say that again? Of course! That’s it!” That’s not literally every episode, but it is basically a structural requirement imposed by the show’s relationship with its audience.
The solution to the mystery depending on figuring out coincidence without the detective merely getting lucky is typically easiest to pull off through exhaustive leg-work—checking every chemist’s shop in a 30 mile radius, that sort of thing. This is why that sort of mystery is most common when the detective is a public detective (i.e. a member of the police) rather than a private detective, or at least when the police and the detective are working together, rather than separately. And even then, Sherlock Holmes had his Baker Street irregulars.
The other approach, which is a compromise that keeps things closer to a detective the reader can relate to, is for the detective to have something to go upon which through intelligence, knowledge, and wisdom allows him to rank coincidental possibilities according to an order they are likely to have happened, and to be right according to a Poisson distribution (basically, they usually get an answer by their third try to verify a coincidence, sometimes it takes a lot of tries, and because no one has infinite effort to give, sometimes they don’t get an answer). Fundamentally this is still the detective getting lucky, but it is a way for the detective to earn his luck. Since the detective doesn’t create the clues but only discovers them, that’s the best he can do in any case.