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The title sets the tone for what may well be the most incredible episode in the history of The Avengers. Steed and Emma are summoned to investigate when several people and a number of huge objects (including an armored tank) seemingly vanish. The cause of these disappearances is a newly developed shrinking machine, which villains intend to use to "miniaturize" their enemies. The silliness reaches its summit when a teeny-tiny John Steed rescues a normal-sized Emma. Written by Philip Levene, "Mission. . .Highly Improbable" was the final episode of the series' sixth season, making its British TV debut on November 18, 1967. Curiously, it was chosen as the first entry in the third American season of The Avengers, on January 10, 1968.