An Idiot Plot is a story in which the conflict at the heart of the story could be solved easily if any one of the main characters would display an ounce of
common sense, or intelligence if you prefer.
Once upon a time, this was considered a horrible literary faux pas, and no proper writer would create such an obscenity. Like the deus ex machina, it just wasn't proper form. Then came television. Then came sitcoms. Situation comedies simply could not exist without idiot plots. "Three's Company" will always be remembered as a show that consisted, for something like five seasons, of nothing but idiot plots.
Many hugely successful Hollywood movies, including a notable percentage of all horror movies, also use idiot plots. "The Blair Witch Project" is a standout example. We are presented with three young protagonists, none of them smart enough to remember that, when lost, if you follow a stream far enough downstream, you will eventually get somewhere. Yes, they are college students from the Big City, so they can be forgiven a few lapses of woodcraft. But their predicament in the movie is entirely their own fault, which is why the audience has no sympathy for them and is not in the least bothered when they all die.
A disclaimer: I actually believe that TBWP is a good, if not great, horror movie. The atmosphere maintained throughout the movie is unbelievably frightening. The filmmakers clearly understand the principles of maintaining suspense, and the cinematography is brilliant, with exactly the right shots jumpy and blurry, adding to the realistic atmosphere. HOWEVER, the plot of the movie is one of the thinnest ever - a classic idiot plot.
"So if the Blair Witch is such a hot item, and it's nothing but an idiot plot, how can you sit there and tell us what a Bad Thing idiot plots are?" you might ask. Go ahead, ask.
Point #1: The idiot plot creates an empty conflict, a problem which is not real. The audience will sense this, and will feel cheated. Knowing that the conflict could easily be resolved, the reader feels no suspense and no involvement with the story.
Point #2: The idiot plot does not allow the audience to empathize with the characters. This is a major problem for the writer. A reader is supposed to identify with the characters. It is almost impossible to care for the characters if they are all blithering idiots.
In the case of the Blair Witch Project, all the greatness is in the technique. TBWP simply revolutionized the way horror movies were visualized, immersing the viewer in what seemed to be a home video of inexplicable horror. Due to its combination of film and video, the changing points of view, and the premise that the footage was actually shot by the actors/characters, the "Project" became something realistically frightening. When a writer can pull off such a show of technique, a certain weakness in the plot may be forgiven.