Deus ex machina

Deus ex machina plural: dei ex machina; English ‘god from the machine’) is a plot device whereby a seemingly unsolvable problem in a story is suddenly and abruptly resolved by an unexpected and seemingly unlikely occurrence, typically so much as to seem contrived. Its function can be to resolve an otherwise irresolvable plot situation, to surprise the audience, to bring the tale to a happy ending, or act as a comedic device. (Wikipedia)