One of the hardest game to try to build and run is the Mystery. A good mystery is easy to write, but very difficult to run, siply because you have no way of knowing what the Players will actually figure out. You don’t want the game to be too easy, or to push theplayers through a script. You also don’t want the mystery to be too difficult. The point of the game is to solve the mystery, so having unintelligble clues is not fun for anyone. One concept that can really help make an exciting yet solvable mystery game is the Three Clue Rule.
This is an excellent article about the Three Clue Rule. I’m linking to the original, rather than rewriting it all. This is the original article:
The Three Clue Rule, written by Justin Alexander
To directly quote the author, “Basically, the Three Clue Rule states: For any conclusion you want the PCs to make, include at least three clues.” Think of each clue as a plan to lead the players to the next step. If they miss Plan A, then Plan B might lead them in the right direction. We all know that no plan will survive contact with the Players, so a variety of solutions to any given problem helps ensure the smooth flow of story.
This helps allow for a much more open game, rather than a scripted plot. Instead of making a plotted story that the Players have to follow, you can set up loactions, events and environments. Detail in the clues rather than relying on dry dice rolls to move things along. One of the troubles with a purely story-driven game is that if the Players miss an important clue, the entire campaign can be at risk, forcing the GM to either railroad the Players thorugh a cut-scene, or simply storytell information instead of the Players playing it out in character. The Players aren’t there to be told a story they’re there to make a story!