A procedure for creating investigation scenarios, or Guess-who adventures

Of all the campaigns I've played, I hold Masks of Nyarlathothep highest. And ever since I played it, I've trying to recreate its atmosphere - without success. The primary reason for this is time. Masks' basic structure is simple: there is a number of nodes, there are local mini-adventures at each node, there are also events that point to the next node, and players gain access to local and the global events through clues. Part of the brilliance of Masks is its scale. The clues are usually very straight-forward: you find a match box from a bar so you know that's where you should go, but the bar is in another country and you also have ten other clues pointing in other directions. What to do? So while the structure is simple, the fundamental feeling of investigating relies heavily on the sheer quantity of leads, locations and events. To create a similar campaign would simply take me too much time.

But another reason is that I wouldn't really know how to start. Internet, and to some extent the game books themselves, is full of advice for creating locations in general, and dungeons and encounters in particular. Other forms of adventures, barring the story-structured adventure, gets much less attention. This is reasonable, since the encounter and the dungeon are widely used adventure building blocks. But this also presents the strange situation where you are given most advice on the things you are already most familiar with. For investigation adventures, you're left with stuff like the three clue rule and that murderers need motivation. And while decent advice, it isn't really helping me. The combined takeaway is that I need three times as many clues, which triples the workload, and need to write some backstory. In other words, I was told to do more of things I already knew I had to do, instead of a different approach to the problem.

So in case your situation is anything like mine, I want to propose a design for creating investigation-centered adventures based on the kid's game "Guess who?". In this design, player-characters are presented with a number of possibilities, and tasked with selecting the correct one. Every step of the investigation leads them closer to zeroing in on the correct solution by eliminating some possibilities, until only the correct one remains.

Below is a procedure for creating such an adventure.

Guess-who adventures

1. Decide on the premise.

The guess-who investigation can only reduce the number of possibilities, never add. This presents a limit to what can be investigated, since there must be some initial separation between possible and impossible cases. Good premises are:
- something is stolen, and only a select group of people knew its location
- a murder on an isolated island
- the gates to the lost dwarven city can only be opened with the correct sequence of runes, but the wrong sequence will flood it with poison gas and seal it off for another century
- have three attempts to enter the PIN code that will open the terrorist's cell phone, and time is running short
- Professor Carlisle disappeared while visiting five ancient cities, where did he go?
- The Crown of Command must be retrieved from the Lich King's hold, but no-one knows in which of his crypts it is
- The Orcs are gathering in alarming numbers in the forest, and the King don't know where to send his troops to meet them

Deciding on the premise is a big deal, since it has consequences for what types of clues are reasonable and how to find them.

2. Set up the clue matrix.

You can make the number of possibilities as large of small as you like. Here I present the basic matrix for 16 possibilities. This makes it solvable with 4 clues, which I think is reasonable.

Criteria 5 and 6 are not needed, and only included here for the sake of completeness. You can save prep time by cutting them.

If your investigation has a lot more starting complexity, like a four-digit PIN, you need a matrix where many more possibilities are excluded than included in each step.

3. Set elimination criteria

If you look at the matrix, you see 8 YESs in each column. One of these is the correct solution, and the other fall into the same category with regards to some criterion. So what are these criteria? Well, really any characteristic that set some of the cases apart from the rest. By knowing that the correct solution fall into the category sharing this characteristic, we can exclude all cases that don't. Therefore, the criteria or characteristics serve the purpose of ruling out possibilities and thereby excluding some cases.

For a murder or theft relevant criteria could be:
Being blonde, leaving the party early, being out in the rain, being elderly, size 10 shoes, and not having an alibi for 9-10 pm.

For a PIN code or dwarven runes, relevant criteria could be:
the second digit is a 3, being a historical date (puts the first two digits at 00 through 20, assuming western calendar), being a prime number, being a birthday of some relation, the four symbols include a combination of frog, ox, moon & sphere.

For a lost city or invasion, relevant criteria could be:
west of some landmark, by a port, far from any major city, site significant in the some personal vendetta with the Lich King, it begins with an N

Four things are important when setting your criteria
1. That they can reasonably be shared between several cases
2. That they are distinct enough that they can be noted separately
3. That they can be formulated as YES/NO.
4. That they exclude a similar amount of cases.

4. Design your clues

Clues can be found in witness reports, letters, travel logs, e-mails or text messages, footprints, a hair left at the crime scene, and so on. Decide if you want the clues to be obvious or require some further reasoning. An obvious clue would be "I saw two people using the ladder that night". A indirect clue would "Smith is in a wheelchair and Hank is afraid of heights; the crime scene can only be reached by a ladder." People feel smarter when they spot an indirect clue, but they are also trickier to set up.

5. Distribute the clues, as you see fit.

Some final comments

Note that the excluded cases in a column don't have to be similar with regard to the characteristic, they just don't match the criterion. For example, your could have 4 suspects with shoe size 7, 4 with size 8, if the footprints indicate that the burglar was size 10.

The investigation will feel more realistic if the proportion isn't strictly half-half regarding each characteristic. Maybe #2 rules out 10 possibilities, while #3 only rules out 7. However, if you shift the proportions, make sure that no clue becomes totally redundant.

Also, since the case can be solved with any four clues, you only need four clues. Including five or six clues, however, give the players more freedom in their investigation. On the other hand, the stucture doesn't really permit more than six clues if they are all to be equal. If for some reason you want more than six clues, the additional clues need to overlap with your original six. A practical way to do this is to think of criteria that by definition or nature appear together. So the size 10 shoe could also lead us to conclude that the burglar was a man, or tall, or both. Or if the invaders will arrive at to the port city, this could also lead us to conclude that come by boat and that the attack is along the coastline. Strictly speaking, these additional clues are of course redundant, but they can provide the players with more options and make the investigation feel more natural.