plot de*vice: (noun) an object or character in a story whose sole purpose is to advance the plot of the story, or alternatively to overcome some difficulty in the plot. (Wikipedia)
Here's the thing: I have no problem with well-done plot devices in film. When Indiana Jones' dad got shot in Indiana Jones and the Last Crusade, forcing Indy into the death trap in search of the Holy Grail, I didn't and still don't bat an eyelash. Even if it were weak (which some might argue the plethora of Nazis was), it would still only be one weak plot device in an otherwise kickass movie.
Not so with Knight and Day, which I made the mistake of watching recently. This movie stunk-with-a-capital-S, due, surprisingly enough, not to Diaz's horrible acting but instead to the lousy writing and absolute dependence on recurring lame plot devices. (SPOILER WARNING, although I doubt anything could spoil a movie that is already the worst movie I've seen since Gothic.)
THE THIRD WORST PLOT DEVICE I'VE EVER SEEN: Is Roy Miller (Tom Cruise) a bad guy or isn't he? Follow along with June Havens (Cameron Diaz) as she tries to find out! (Well, all right, I can't complain about this too much since it played a part in my beloved Princess Bride, but if you're going to go this route, the rest of the movie should be as yummy as Cary Elwes' bare chest in the Pit of Despair.)
THE SECOND WORST PLOT DEVICE I'VE EVER SEEN: In the second half of the movie, June suddenly develops some kind of skill at hand-to-hand fighting (which she's never even hinted at before) in order that she can roll around with Roy in what were undoubtedly meant to be sexual tension-laden scenes but instead induced only near-fatal eye rollings.
THE WORST PLOT DEVICE I'VE EVER SEEN: Finally, what was unforgivable even to this "please let me suspend disbelief" junkie, throughout the film, Roy Miller drugs June Havens in an effort to (a) move the plot to another continent in a short period of time, and (b) to show brief flickers of dangerous situations without the high cost of special effects. It came off as lame and cliche, not wry and ironic.
I guess my point is, if you're a Hollywood screenwriter, go easy on the plot devices. And if you're not a Hollywood screenwriter, don't ever watch Knight and Day.