the.com/plot device

means An element inserted into a story mainly to make the plot move forward, rather than because it grows naturally from character or realism.

from A 20th-century term built from plain parts: 'plot' (the arranged sequence of events in a story, from an older sense of 'plot' as a plan or scheme) and 'device' (a contrivance or trick, from Old French 'devis' meaning a plan or design). Literary and film critics started using the combined phrase to call out moments — a conveniently found letter, a sudden case of amnesia — where the machinery of storytelling shows through, like seeing the gears behind the clock face.