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RPG Evolution: The Fair-Play Whodunnit

The rules for a fair-play whodunnit can make for a fun investigative adventure.

The rules for a fair-play whodunnit can make for a fun investigative adventure.


Picture courtesy of Pixabay.

Who Done What?​

Although they've since fallen out of vogue, fair-play whodunnits were quite popular with early detective fiction of the 1930s. The term refers to mystery stories where the reader or audience is given all the necessary clues and information to solve the crime alongside the detective. In a fair-play whodunnit, the emphasis is on the puzzle-solving aspect of the mystery. The term "fair-play" indicates that the mystery adheres to a set of rules that ensure the reader has a fair chance of deducing the identity of the culprit before the big reveal.

Fair-play whodunnits lend themselves to role-playing campaigns because they are meant to be solvable. The detectives, whether amateur or professional, must use their intellect, observational skills, and logical reasoning to piece together the clues and unravel the truth behind the crime. The story presents a series of tantalizing clues, hidden motives, and potential suspects, inviting players to engage in the investigation themselves.

The Ten Commandments of Fair-Play​

In 1928, the writer Father Ronald Knox created a list of plot devices known as Knox's Decalogue. These rules still apply today, but require some tweaking to apply them to fantasy campaigns.
  1. The Criminal: In the Fair-play Whodunnit, the culprit must be introduced early in the story, but their inner thoughts should remain hidden from the reader or players. In a D&D campaign, DMs can introduce NPCs who become suspects in the mystery. However, the true perpetrator should be hidden, with their motives obscured until the climactic reveal.
  2. No Supernatural Agencies: The use of supernatural or preternatural elements is discouraged in Fair-play Whodunnits. In a fantasy D&D setting, this translates to avoiding the use of overtly magical means to commit the crime. Instead, focus on mundane methods amplified by magical twists or disguise spells. In other words, internal logic is a must -- if a spell is used, the PCs should have access to it or known how it's cast. Some older movies exercise several plot contortions to avoid breaking this rule, but for fantasy campaigns the game's rules provide something of a guideline. Conversely, a villain shouldn't use a spell that isn't in the rules.
  3. Limited Secret Passages: Fair-play whodunnits relied heavily on sneaking around, providing seemingly magical routes for villain to appear and disappear from the screen. While only one secret passage is allowed in the Fair-play Whodunnit, DMs can adapt this rule by introducing enchanted doorways or teleportation circles within suitable locations. And remember, if the villain can use it, so can the PCs!
  4. No Undiscovered Poisons: Like secret passages, mystery authors were fond of adding undetectable, instantly lethal poisons to their plots to make it nearly impossible for authorities to determine the identity of the killer. In a fantasy setting, DMs can use rare or magical poisons that already exist in the rules, but should avoid creating new poisons that nobody's ever heard of.
  5. Avoid Clichés and Stereotypes: The original Knox's Decalogue prohibits the use of certain ethnic stereotypes. In a D&D campaign, avoid using fantasy equivalents, such as clichéd species, to prevent perpetuating harmful tropes. This is often played against type (everyone suspects the dwarf of poisoning someone, or the elf of casting a spell).
  6. No Detective Accidents: Ensure that the detective's discoveries are the result of clever deduction and not fortunate accidents. In a D&D campaign, the detective skills of a player character should lead to the uncovering of vital clues rather than coincidental discoveries -- or DM fiat.
  7. No Detective as the Culprit: As in the original rules, the detective cannot be the culprit in a Fair-play Whodunnit. In a D&D campaign, this means keeping player characters free from suspicion, allowing them to focus on solving the mystery. It's tough to make them out to secretly be villains in any case.
  8. Instant Clue Reveal: Clues should be readily available for players to inspect. In a D&D campaign, ensure that players have access to all the clues needed to solve the mystery, but challenge them with cleverly hidden or magically disguised information.
  9. The NPC's Mind: The "Watson" character, typically a foil for the lead detective, should be open and transparent in their thoughts. In a D&D campaign, this can be an NPC or a player character who assists the party. Their capabilities should be slightly below the average player's, allowing players to feel engaged and challenged by the investigation.
  10. Prepare for Doubles: In a fantasy setting, "doubles" can take the form of doppelgangers or magical clones. Ensure that their existence is foreshadowed to avoid surprising players with unexpected duplications.
We discuss fifty of these types of dramas in our recently-released podcast, 50 Date Night Screams, along with D&D statistics for each major villain. Although these rules are decades old, they still apply to fashioning a mystery that heroes (and by proxy, their players) can figure out on their own.

Your Turn: What guidelines do you use for solving mysteries in your game?

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Michael Tresca

Michael Tresca


Follower of the Way
I did more or less this (much less formally, of course) with a whodunnit mystery. The one "not quite there" thing I did was that I said the victim--whom the party Ranger(with Cleric spells) revived--wouldn't be able to help identify the killer because the manner of their death had scrambled their memories of the time before said death. (Also, because the player rolled only a partial success, it would take the victim time to fully revive.)

There were clues hidden in plain sight, and difficulties to overcome. For example, I leveraged the fact that one of my players studied physical anthropology. Player knows that bodies take time before they achieve livor mortis--the pooling of the blood in the lower extremities. But the body, mere minutes after being found with a knife in its back, had not bled all over the place. Indicating, as this player (and character) well knew, that the body had been staged and the death occurred several hours earlier. But that was contradictory information--people had seen the victim at the masquerade ball only an hour before, getting into a row with a visiting dignitary! Clearly all was not as it seemed...and someone had the ability to disguise themselves as the victim in order to throw them off the scent.

Things proceeded from there and the players were able to get all of the information and catch the true culprit. All in all, an excellent little adventure along the way. I haven't done any "murder mystery" type adventures since--feels like it would be a bit repetitive if I did. But there are other investigations that can be similarly "whodunnit" without being murders.

In addition to Knox's Decalogue, I would be remiss as an Umineko fan if I didn't also recommend the (significantly more verbose) Twenty Rules for Writing Detective Stories by S. S. Van Dine:

  1. The reader must have equal opportunity with the detective for solving the mystery. All clues must be plainly stated and described.
  2. No willful tricks or deceptions may be placed on the reader other than those played legitimately by the criminal on the detective himself.
  3. There must be no love interest. The business in hand is to bring a criminal to the bar of justice, not to bring a lovelorn couple to the hymeneal altar.
  4. The detective himself, or one of the official investigators, should never turn out to be the culprit. This is bald trickery, on a par with offering someone a bright penny for a five-dollar gold piece. It's false pretenses.
  5. The culprit must be determined by logical deductions - not by accident or coincidence or unmotivated confession. To solve a criminal problem in this latter fashion is like sending the reader on a deliberate wild-goose chase, and then telling him, after he has failed, that you had the object of his search up your sleeve all the time. Such an author is no better than a practical joker.
  6. The detective novel must have a detective in it; and a detective is not a detective unless he detects. His function is to gather clues that will eventually lead to the person who did the dirty work in the first chapter; and if the detective does not reach his conclusion through an analysis of those clues, he has no more solved the problem than the schoolboy who gets his answer out of the back of the arithmetic.
  7. There simply must be a corpse in a detective novel, and the deader the corpse the better. No lesser crime than murder will suffice. Three hundred pages is far too much pother for a crime other than murder. After all, the reader's trouble and expenditure of energy must be rewarded.
  8. The problem of the crime must be solved by strictly naturalistic means. Such methods for learning the truth as slate-writing, ouija-boards, mind-reading, spiritualistic seances, crystal-gazing, and the like, are taboo. A reader has a chance when matching his wits with a rationalistic detective, but if he must compete with the world of spirits and go chasing about the fourth dimension of metaphysics, he is defeated ab initio.
  9. There must be but one detective - that is, but one protagonist of deduction - one deus ex machina. To bring the minds of three or four, or sometimes a gang of detectives to bear on a problem, is not only to disperse the interest and break the direct thread of logic, but to take an unfair advantage of the reader. If there is more than one detective the reader doesn't know who his co-dedutcor is. It's like making the reader run a race with a relay team.
  10. The culprit must turn out to be a person who has played a more or less prominent part in the story - that is, a person with whom the reader is familiar and in whom he takes an interest.
  11. A servant must not be chosen by the author as the culprit. This is begging a noble question. It is too easy a solution. The culprit must be a decidedly worth-while person - one that wouldn't ordinarily come under suspicion.
  12. There must be but one culprit, no matter how many murders are committed. The culprit may, of course, have a minor helper or co-plotter; but the entire onus must rest on one pair of shoulders; the entire indignation of the reader must be permitted to concentrate on a single black nature.
  13. Secret societies, camorras, mafias, et al. have no place in a detective story. A fascinating and truly beautiful murder is irredeemably spoiled by any such wholesome culpability. To be sure, the murderer in a detective novel should be given a sporting chance; but it is going too far to grant him a secret society to fall back on. No high-class, self-respecting murderer would want such odds.
  14. The method of murder, and the means of detecting it, must be rational and scientific. That is to say, pseudo-science and purely imaginative and speculative devices are not to be tolerated in the roman policier. Once an author soars into the realm of fantasy, in the Jules Verne manner, he is outside the bounds of detective fiction, cavorting in the uncharted reaches of adventure.
  15. The truth of the problem must at all times be apparent - provided the reader is shrewd enough to see it. By this I mean that if the reader, after learning the explanation for the crime, should reread the book, he would see that the solution had, in a sense, been staring him in the face - that all the clues really pointed to the culprit - and that, if he had been as clever as the detective, he could have solved the mystery himself without going on to the final chapter. That the clever reader does often thus solve the problem goes without saying.
  16. A detective novel should contain no long descriptive passages, no literary dallying with side-issues, no subtly worked-out character analyses, no "atmospheric" preoccupations. Such matters have no vital place in a record of crime and deduction. They hold up the action, and introduce issues irrelevant to the main purpose, which is to state a problem, analyze it, and bring it to a successful conclusion. To be sure, there must be a sufficient descriptiveness and character delineation to give the novel verisimilitude.
  17. A professional criminal must never be shouldered with the guilt of a crime in a detective story. Crimes by house-breakers and bandits are the province of police departments - not of authors and brilliant amateur detectives. A really fascinating crime in one committed by a pillar of a church, or a spinster noted for her charities.
  18. A crime in a detective story must never turn out to be an accident of a suicide. To end an odyssey of sleuthing with such and anti-climax is to hoodwink the trusting and kind-hearted reader.
  19. The motives for all crimes in detective stories should be personal. International plottings and war politics belong in a different category of fiction - in secret-service tales, for instance. But a murder story must be kept gemuetlich, so to speak. It must reflect the reader's everyday experiences, and give him a certain outlet for his own repressed desires and emotions.
  20. And (to give my Credo an even score of items) I herewith list a few of the devices which no self-respecting detective-story writer will now avail himself of. They have been employed often, and are familiar to all true lovers of literary crime. To use them is a confession of the author's ineptitude and lack of originality. (a) Determining the identity of the culprit by comparing the butt of a cigarette left at the scene of the crime with the brand smoked by the suspect. (b) The bogus spiritualistic seance to frighten the culprit into giving himself away. (c) Forged fingerprints. (d) The dummy-figure alibi. (e) The dog that does not bark and thereby reveals the fact that the intruder is familiar. (f) The final pinning of the crime on a twin, or a relative who looks exactly like the suspected, but innocent, person. (g) The hypodermic syringe and knockout drops. (h) The commission of the murder in a locked room after the police have actually broken in. (i) The word-association test for guilt. (j) the cipher, or code letter, which is eventually unraveled by the sleuth.

Which reminds me, I need to go see a witch about some gold...


Follower of the Way
I think the biggest sin in may adventures, mystery or otherwise, is the "Roll to Countinue". Something vital requires the players succeed on a roll otherwise the adventure can't be completed. This might be a clue, a secret door, tracks, etc.
Frankly, it's one of the bigger sins in GMing generally.


Follower of the Way
Rule 7 and 9: I guess no one told Agatha Christie...
It's worth noting, she only did #7 once, and it was rather clearly a major subversion. It received good reviews, to be fair, but it's also a controversial move.

Not sure when she violated #9 though? Unless you mean the same story, which would mean it couldn't be #7 (since the Watson-alike, the Detective's Assistant, is distinct from the Detective character.)

It's worth noting, she only did #7 once, and it was rather clearly a major subversion. It received good reviews, to be fair, but it's also a controversial move.
Not sure when she violated #9 though? Unless you mean the same story, which would mean it couldn't be #7 (since the Watson-alike, the Detective's Assistant, is distinct from the Detective character.)
Spoiler: a certain long running play that I'm not allowed to talk about.

Gladys Mitchell broke that one too.

Rule 7 and 9: I guess no one told Agatha Christie...

Additionally, the Agatha Christie book in question (or the predominant one that comes to mind...my collection is far from complete) - The Murder of Roger Ackroyd - was published in 1926, while Knox's Decalogue is generally dated to around 1929 (and Van Dine's Rules to 1928), so it's likely that on some level those rules were created in response to Christie, rather than Christie deliberately subverting them.
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