Deus ex machina

"The phrase deus ex machina (literally "god out of a machine") describes an unexpected, artificial, or improbable character, device, or event introduced suddenly in a work of fiction or drama to resolve a situation or untangle a plot (e.g. an angel suddenly appearing to solve problems).
The phrase has been extended to refer to any resolution to a story that does not pay due regard to the story's internal logic and is so unlikely that it challenges suspension of disbelief, allowing the author to conclude the story with an unlikely, though more palatable, ending.
In modern terms the deus ex machina has also come to describe a being, object or event that suddenly appears and solves a seemingly insoluble difficulty, where the author has "painted the characters into a corner" that they can't easily be extricated from (e.g. the cavalry unexpectedly coming to the rescue, or James Bond using a gadget that just so happens to be perfectly suited to the needs of the situation)." (Wikipedia)