Originally posted February 2009

Last year I found a good thread on writing mysteries for RPGs. The advice was for games that follow a model more typical of crime shows (investigators find crime, make theories, test them, make new theories if wrong) than of the typical RPG model (investigators find clues, follow them to next scene, find more clues, etc.). Below are my summarized notes on the thread, rephrased and expanded a bit with my own thoughts.


  • Deduction: Given a known case and a rule related to the case, the result of the case can be determined. (Given that unknown beans are from a bag, and that all beans in that bag are white, the unknown beans must be white.) This is the strongest type of investigation, but requires most of the information before reaching a solution.
  • Induction: Given a known case and the result of the case, the case’s rule can be determined. (Given that these beans were taken from the bag and all of these beans are white, all beans in the bag must be white.) This is the second strongest type of investigation, because reaching a solution generally allows the solution to be further tested.
  • Abduction: Given a result of a case and a rule applicable to the case, the case itself can be determined. (Given that these beans are white and all beans in that bag are white, these beans must be from that bag.) This is the weakest type of investigation, because it proves nothing, merely suggests a solution that can be tested in other ways. However, since investigations will often start with a result (the evidence of the crime) and rules (forensic sciences) but will not initially include the case (the method and motive of the crime), abduction is the standard investigation method.

In order to strengthen abduction you can turn to deduction, induction, or comparison:

  • Deduction: If the abduced case is true, using other rules what other result must be true? (If the beans are from this bag, given that there are a standard number of beans in a bag, the bag must be missing this number of beans.)
  • Induction: If the abduced case is true, using other results what other rules must be true? (If the beans are from this bag, and the bag currently contains a certain number of beans, these beans plus the beans in the bag must equal the original number of beans in the bag.)
  • Comparison: If the abduced case is true, no other adduced case can adequately explain all the results. (Given that these beans are white and all beans in that bag are white, are there any other bags of white beans from which these beans could have originated?)

Given all the facts (clues) of the result, what case might have been true to exactly produce all those facts?

Mystery case blocks/stages:

  • Stage 1, The Scene of the Crime: Characters investigate the result, finding all details however trivial. (Designer must place all essential clues to describe the result and ensure they are discovered.)
  • Stage 2, Abduction: Characters discuss, adducing a small number of cases that would explain the result and depend on certain or likely rules. (Designer encourages them to find as many solutions as possible and grade them on how well they make the known fact predictable, and encourages them to find the simplest hypothesis.)
  • Stage 3, Investigation: Characters investigate possible violations of the adduced cases, eliminating rules that turn out to be unsupported by additional evidence, and looking for additional, non-obvious clues that would support a particular case. (This is the major part of the game, and lots of new evidence will be uncovered, some of it superfluous. At the end of the block, the characters should be very close to a correct case.)
  • Stage 4, the Case: Characters rebuild the remaining cases until they can eliminate all but one, repeating the third block until only a single, watertight case remains. (Designer should make it hard to get to block 5 with an incorrect case, instead funneling the characters back into block 3 if there are still holes. In murder cases, killing a suspect or another murder in a way that gives a suspect an alibi is the usual method for breaking an incorrect case.)
  • Stage 5, the Plan: Characters develop a plan of action that will confront the culprit of the final case in a way that proves the case and produces a climactic scene.
  • Stage 6, the Climax: Characters deal with the fallout of the fifth block, possibly using previously determined information to fight or track a culprit that has fled after the revelation of block 5.

Episodic blocks:

  • Teaser: Introductory material and first beat of the discovery
  • Act 1: Block 1, likely ending on a hard-won clue
  • Act 2: Blocks 2 through 4, likely ending on the revelation of the case
  • Act 3: Block 5, likely ending on the villain raising the stakes
  • Act 4: Block 6, final confrontation with the villain/climax and denouement

Four or five major beats per act.

Inventing the Case:

Step 1: Straight Story

Who – Who did it and who was hurt?
What – What was the crime?
When – When was it carried out?
Where – Where was it carried out?
Why – Why was it carried out?
How – How was it carried out?

What, Where, and the second Who should be immediately obvious, with the When following shortly unless concealing the time is important (and a range of times should still be apparent). The How should become evident during the intial investigation. The first Who and the Why are identified by the investigators.

Step 2: Messiness

What clues were left behind as a result of the crime not being perfect? Advanced: were there any coincidences that covered up clues?

Step 3: Bystanders

Who was connected to the victim and can provide information relevant to the crime? Who gains and who loses and who knows about it? Does he have enemies or people that feel strongly about the crime?

Step 4: The Result

What is the exact timetable of events? Who was there and who was nearby? What basic clues were left, and how are they to be found? These are essentially points that the police will likely find with a few hours, if they take an interest.

Every NPC that is connected to the victim or location but not involved in the crime should have a routine or an alibi that eventually eliminates him or her as a suspect. Any actions the criminal took to distract the NPC should become obvious as a deviation from the NPC’s intended routine.

Step 5: The Difficulty

Why can’t the case simply be left up to the police to solve?

Step 6: The PCs

How do the PCs get involved? If they’re called in, who calls them and why? If the person that called them in has a theory as to what happened, it’s probably wrong.

Rule of 7:

No more than seven new named NPCs, important clues, or major ideas. Seven key concepts total is even better.

Edit (7/2010): Rob Donoghue has some excellent advice on different types of murder mysteries that syncs nicely with the information above.