There's no two ways about it - a plot twist can either make a break a movie in a single moment.
A great twist will enrich a film's themes and leave viewers blown away by the ingenuity of it all, while a poor twist will feel cheap, unearned, and like nothing more than a lazy attempt to shock the audience.
But whether good or bad, plot twists have a tendency to stick in viewers' minds, especially those that go the extra mile to get really weird, defy expectations, and maybe even leave the audience completely traumatised.
And so, these 10 horror films boast arguably the most shocking, unexpected, creative, and otherwise WTF twists you're ever likely to see.
The likes of Psycho and The Sixth Sense didn't make the cut because they're just so familiar in 2021, but these movies nevertheless dared to take a hard swing for the fences, no matter how it all turned out.
They may not all rank among the best twists the genre has to offer - though some of them certainly do - but they're absolutely the most bold and devilishly unexpected...
13. Jamie Stabs Darlene - Halloween 4: The Return Of Michael Myers
Halloween 4 may not be a very good movie - if we're being kind, and we are - but it does feature one of the series' boldest and most memorable endings.
After Michael Myers is attacked by a lynch mob and buried in a mine shaft, all seems to be back to normal, as Michael's young niece Jamie (Danielle Harris) is reunited with her foster parents.
But in a hugely unexpected twist, the final scene sees Jamie's foster mother Darlene (Karen Alston) stabbed while running a bath for Jamie, before the culprit is revealed to be Jamie herself, brandishing a pair of scissors just as Michael did in the very first Halloween.
The obvious implication is that Jamie has "inherited" her uncle's murderous instincts, though this was sadly jettisoned in the next sequel, where Darlene is revealed to have survived and Jamie is admitted to a children's clinic for her psychological issues.
Oh, and she develops a telepathic link with Michael. Right.
Still, for a moment this twist ending posited an interesting gear-shift for the series, albeit one which was ultimately nothing more than a hollow, desperate attempt to lure fans back for the next movie.