Story structure
Plot twist
A literary technique that introduces a radical change in the direction of the plot, usually foreshadowed but delivered with an element of surprise.
A plot twist is a literary technique that introduces "a radical change in the direction of the plot" of a work of fiction. A twist may be foreshadowed "to prepare the audience to accept it," but "it usually comes with some element of surprise"; revealed in advance, it is a spoiler. The device is old: the Arabian Nights tale "The Three Apples" is the early example usually cited.
The mechanics are a catalogue of familiar devices in twist form: the recognition scene, the sudden reversal of fortune, the false protagonist and the red herring, planted to steer readers toward a wrong conclusion. The craft test sits in the hinge between the story's two halves: a good twist recontextualizes what came before without contradicting it, which is why planting matters more than the reveal. See foreshadowing.