Deus ex machina

n. someone or something which unexpectedly comes to the rescue at the last moment (especially in a book or play)

The phrase deus ex machina (Latin (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).
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Noun
1. any active agent who appears unexpectedly to solve and insoluble difficulty
(hypernym) causal agent, cause, causal agency

deus ex machina, someone or something which comes to the rescue at the last moment

n. deus ex machina, someone or something which comes to the rescue at the last moment