Only One Plausible Suspect
Due to mystery fiction conventions, there's only one character who can turn out to be the culprit.
The story involves 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 who the guilty party is. 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, such as kids or animals. Of course, even if the audience can guess who did it, it isn't as easy for the protagonists to solve the mystery, since 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. Warning: the examples below contain spoilers!
- In Die Another Day, we find out there's a mole inside the MI6 who has, among other things, informed the bad guys who James Bond is. Now, obviously the mole can't be Bond himself, nor M or Q or Moneypenny, as they are all mainstays of the franchise. Besides them, there is only one other major MI6 character in the movie, who − surprise, surprise! − does turn out to be the mole.
- In Captain America: The Winter Soldier, Steve Rogers sets out to discover who has ordered the attacks on both him and Nick Fury. Only one plausible suspect is shown, Alexander Pierce, especially after he makes several vaguely ominous statements like "to build a better world sometimes mean tearing the old one down". Perhaps in awareness of this, the film makes little attempt to disguise Pierce as the culprit and it only takes five minutes after he and Steve meet for him to become a visible antagonist.
- In the Doctor Who story The Deadly Assassin, the President of the Time Lords is assassinated, and the Doctor framed for the crime. The only suspect who gets any significant amount of screen time is Chancellor Goth, one of the candidates-in-waiting for the Presidency, and he turns out to be the real assassin.
- Towards the tail end of Torchwood: Miracle Day we're told there's another evil Mole in the CIA other than the Obviously Evil, Fat Bastard that was outed earlier. There are three CIA operatives that aren't part of the main cast. One is Da Chief and working pretty diligently with Torchwood and wants the case solved and the mole outed. The other is developing the software to find the mole. That leaves the last one, who was featured prominently in the first episode, who we had just been reminded exists the episode prior, and who just had her wardrobe change up to low cut cleavage showing dresses. To be fair though, the audience is let in on it before the heroes, who were busy juggling the Idiot Ball the entire season.
- Subverted in Sleuth (as well as its movie adaptations): the first half of the story features only two characters, and when one of them disappears under suspicious circumstances, it would seem the other one has killed him. But it turns out he never disappeared at all, and the policeman investigating the case is actually the "disappeared" character in disguise.
- This trope is used frequently in various versions of Scooby-Doo. Subverted in one episode of What's New, Scooby-Doo?, though: the crook in "It's All Greek to Scooby" turns out to be some random person the gang has never met beforehand. Velma naturally is rather displeased and keeps insisting her theory about who was the monster was at least plausible.
Hello, Unknown Troper. You'll need to get known to lend a hand here.