In Top Hat, Ginger Rogers shuns the ardent Fred Astaire because she thinks he’s her best friend’s husband. How she could persist in this belief for any length of time is never explained — the misunderstanding drives the whole story.
This is an “idiot plot,” defined by Roger Ebert as “a plot which is kept in motion solely by virtue of the fact that everybody involved is an idiot.”
The term was coined by science fiction author James Blish, whose colleague Damon Knight added the second-order idiot plot, “in which not merely the principals, but everybody in the whole society has to be a grade-A idiot, or the story couldn’t happen.”
Such contrivances are annoying, but we’ll forgive a lot if we get to watch Fred Astaire dance. “How is it that Ginger has never met her best friend’s husband?” asks critic Alan Vanneman. “Well, Europe is a big place.”