An idiot plot was first defined by James Blish as a plot that only hangs together because all of the main characters act like idiots. I’d add “…when they’re not supposed to be idiots,” because there are plenty of effective (usually comic) stories where the point is that the main characters are supposed to be idiots. The Three Stooges, for instance. Or Bertie Wooster.
A true idiot plot isn’t supposed to be funny, and it isn’t usually a matter of a single stupid mistake or decision on the part of a couple of main characters (though if the mistake is big enough, it can happen). It’s a matter of supposedly intelligent characters overlooking the obvious, making foolish decisions, and ignoring better choices over and over again, without any plausible justification.
Let me repeat that in slightly different words: one giant plot hole does not an idiot plot make, nor does one stupid character, nor does a character or set of characters who are obviously supposed to be feckless and inattentive. An idiot plot is when you have five supposedly-sensible characters sitting around trying to come up with a plan for getting into the fortress, and the very first thing they all agree is that they should not try to trick the guards, and ten pages later the plan they’ve come up with is to trick the guards anyway, when it is perfectly obvious to the reader that based on what they’ve said, they don’t need to get into the fortress at all. And this sort of thing happens over and over.
Every once in a while, you run across a single mistake that is enough to turn a story into an idiot plot all by itself. Usually, it is something that’s a major turning point in the plot that the average reader just can’t buy. For instance, I read a story a while back in which the heroine was supposed to be very smart. The story opened with this woman explaining to her partner why she could not, under any circumstances, contact her sister in any way, for fear of leading the drug lords they were chasing to her innocent family. The heroine had, in fact, sent the sister off into hiding with cleverly designed, multiply-redundant safety factors, and no one but the heroine knew where the sister was.
OK, at this point it was already obvious, given the tropes of modern fiction, that something was going to go wrong and the villains were somehow going to find the sister. The question was, how? The heroine had done a really good job of hiding her, so about all I could think of was the heroine breaking under torture.
No such thing. In mid-book, the heroine’s partner is killed and she is so distraught that all she can think of is going to find her sister and crying all over her. She forgets all about the careful safety factors and redundancies and so on, and buys a plane ticket in her own name and goes straight to where the sister is living, without making any effort at all to throw off the villain’s henchmen that she knows are tailing her and without telling anyone where she is going. Naturally, the villain follows, captures them both, torture and questioning, etc., followed by the heroine on a massive guilt trip for leading the villain straight there, which provides much of the character development for the last half of the book.
The rest of the plot was fine, but so much of it hung on this one incident that I’d have to call the whole thing an idiot plot. I suspect, though I have no way of knowing, that the author had a really clear vision of the heroine’s guilt trip in the last part of the book. Unfortunately, the author didn’t have the heroine make the kind of mistake that a careful, smart, planning-ahead sort of person would make. I might even have bought it if the writer had spent a few more paragraphs on the heroine’s emotional breakdown, and made me believe that this woman would suddenly go to pieces to such an extent that she would completely ignore her sister’s safety out of her own need for comfort, but as it was…no.
I don’t know whether coming up with a different mistake or better reasons was too hard, or whether the author was deeply afraid of committing purple prose during the breakdown scenelet, or whether it was so obvious in the author’s head that the things that made the heroine’s actions believable just never got down on paper, but whatever it was, that one incident pretty much wrecked the book for me.
The three most common times this sort of thing happens are 1) when the author is running their plot by-the-numbers (the plot outline says the hero does X, so that’s what the hero does, whether it is in character or not), 2) when the author is so involved in the next scene and writing towards it in such a white-hot heat that he/she never stops to consider that the really plausible thing for the character to do would be to take a cab home from the restaurant and never get mugged in the alley at all, and 3) when the author has everything so clear in his/her head that the parts that make things believable and reasonable never make it onto the page.
A good critique group or a good batch of first-readers can head all of these off at the pass. Failing that, the author has to stop at some point and think very hard about all the possible moves the character could make, and which of them the character would be most likely to actually do, regardless of what the author has planned. If there is anything that is a smarter, more reasonable, more obvious choice of action, the author has two choices: have the main character do that, and ditch the plot as planned (no matter how much in love with it the author is), or else take the action the author wants the protagonist to take and come up with some setup or event that will make it the most reasonable choice. (I find geography very useful for this; a convenient impassable mountain range, swamp, or river can force characters to take the only remaining route, which they would otherwise be far too sensible to go anywhere near.)