A mystery where, because of how fiction works, only one character can be the culprit.
Should We Have This? Is This Tropable? Needs More Examples. Needs a Better Description. Note: This is only a preliminary draft, as I'm not completely sure if this is a true trope, an at the moment I have only one example... So I wanted to hear more comments on the subject, and see if more examples come up, before writing a fuller description. You have a mystery, where someone has committed a crime or misdemeanor of some sort, but neither the protagonist(s) nor the audience is supposed to know the guilty party. However, The Law of Conservation of Detail and the rules of Fair Play Whodunnit state that the culprit must be a character who appears in the story before The Reveal. It can't be someone the audience has never seen before, and if the mystery is a big part of the plot, it can't really be a minor background character either. It has to be someone important. In Fair Play Whodunnits and many other types of Mystery Fiction, the writers usually introduce several potential suspects to the crime, and in the end one of them is found to be guilty, while the others turn out to be mere red herrings. However, in some pieces of fiction (typically ones where the mystery isn't the main driving force of the plot), there are no red herrings, and the audience can rather easily deduce the culprit, since he's the only possible major character who could have done it. Either there are no other significant characters among the suspects, or all the other major characters can be ruled out because they're the protagonists, series regulars (in the case of serial media), or other types of characters that aren't typically used as a culprit. Of course, even if the audience can guess who did it, it isn't as easy for the protagonists to solve the mystery, as for them the guilty party could be any minor character, or even someone who doesn't appear in the story at all. It's only the audience who can rule these people out. If the writers don't care about the rules mentioned above, they can make the culprit turn out to be some completely unexpected minor character, or even someone we've never met before, but these kind of mysteries tend to be much rarer than the ones that follow the rules. This trope can overlap with Chekhov's Gunman, if the guilty character doesn't seem to have any proper function in the story before The Reveal.