of Narrow Margin (1990)
Originated by SF author James Blish and popularized by film critic Roger Ebert during his review of the remake of Narrow Margin, this trope is a term for a Plot that hangs together only because the main characters behave like idiots. A single intelligent move or question by any of the characters, and all problems would be resolved. It's not so bad if the characters are supposed to be acting like idiots, but it's very bad if the Idiot Plot depends on intelligent characters picking up the Idiot Ball for the plot to work.
Even worse than that is the "Second-Order Idiot Plot", in which the plot can only function if the world population suddenly loses about 50 IQ points. In fact, author Damon Knight originally coined the term "second-order idiot plot" to refer to a science fiction story that features a fictional society that can only exist if everyone living there is an idiot.
Idiot plots can often be avoided with a simple wave of the hand. If the audience would have spent the entire story wondering why the hero didn't try some obvious tactic, a hand wave at the beginning of the story as to why that wouldn't work would prevent an idiot plot, regardless of how contrived the excuse was. However, if the hand wave is bad, it may actually create a new obvious solution just as bad as the original.
It is important to note that this is not always a bad thing, and is sometimes the entire point of the story. Part of what can make a drama so dramatic or a tragedy so tragic is that the characters truly could have avoided it if they had acted rationally instead of irrationally. (If the characters come to realize this In-Universe, by the way, that's a "Could Have Avoided This!" Plot.) Likewise, this is frequently Played for Laughs in a comedy or satire, either to laugh at the antics of the idiots or make a point about the idiocy in society. This is also very justifiable if most of the cast is not in a position where they can make rational or informed decisions, such as if they're young, poorly educated, or under stress. And sometimes it's just a case of Reality Is Unrealistic; many real-world problems, viewed from a distance with plenty of hindsight and no time pressure to solve the issue now, can seem like Idiot Plots themselves, but the rub is that people at the center of the problem may not have those advantages. Additionally, one thing to take into account is that we are viewers, for the characters, this is reality.
Compare Just Eat Gilligan for when everyone's guilty of the single idiocy of ignoring something that could resolve the plot. See also Fourth Wall Myopia - when the audience fails to recognise the difference between themselves and the characters. You may have seen the Obviously Evil person twirling their mustache at the camera, but the characters who get fooled did not.
