An adjective denoting something that is, or is closely related to a plot device. The phrase is used to make plot devices blatantly obvious for what they are, and thus can be considered to be of significance to humor that intentionally breaks the fourth wall.
John leapt into the vat of radioactive waste, which plot-devicedly gave him super powers instead of radiation poisoning.
The heel of Mary's shoe suddenly snapped, sending her to the floor and plot-devicedly allowing her to evade the killer's axe.
The staggering blow would have been fatal to any normal human, but plot-devicedly enough Jack was the last survivor of an ancient race and the magic in his blood kept him alive; though removing all conflict from the story in doing so.