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Someone has broken into an "impenetrable" underground spy headquarters and microfilmed a valuable document. The subsequent investigation reveals the startling "fact" that John Steed was in on the caper. While Steed languishes in prison, Cathy tries to get at the truth by trailing the beautiful female spy (Edina Romay) with whom Steed was apparently in cahoots. Written by Philip Chambers, "The Nutshell" was originally telecast in England on October 19, 1963; the episode wasn't seen in America until March 8, 1991.