I've been considering running a game based on Murder on the Orient Express (but steampunk). I ran across a post that brought up a point about the mystery genre, in RP. What happens when you fail?
Honestly, when you look at the mystery genre in other places, the main characters "win" because they solve the crime. They figure out "who done it". They save the day. However, what happens if the dice just don't roll in favor of your players? What if they don't connect clues? On the other hand, if the clues are too obvious, then there's really no challenge or game aspect to the mystery RP.
First, I think for a mystery game to work, like any genre of game, you need to have a group who enjoys working out clues. Some groups don't like puzzles. Perhaps the way to solve this problem is to have clues be found if they get through the obstacles (a difficult social situation or a physical obstacle) but make the connecting difficult.
Also, your characters don't have to solve it. Perhaps they miss something and the criminal strikes again next session. Maybe they follow a red herring. Maybe there's a time limit before someone else gets killed. I was recently in a game where our PCs ran away in horror, rather be murdered or solve the mystery. We still don't know what was going on in that house, and I think in RP that can be okay.
Like to GM in Theatre Noir? If this discussion about inspiration has spurred new ideas, perhaps you want to playtest GM Theatre Noir.
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