The Avengers.S06E18.The Morning After

A double-agent, codenamed Merlin, steals a new sleep gas and tries it out on Steed. Awaking 24 hours later, with Merlin his prisoner, Steed can find no one to hand him over to: everywhere he goes the streets are completely deserted.
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"The Morning After" is the eighteenth episode of the sixth series of the 1960s cult British spy-fi television series The Avengers, starring Patrick Macnee and Linda Thorson. Its first broadcast was in the ATV region of the ITV network on Friday 10 January 1969. Thames Television, who commissioned this series of the show for ITV, broadcast it in its own region 19 days later on Wednesday 29 January. The episode was directed by John Hough, and written by Brian Clemens.
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Cast
Patrick Macnee as John Steed
Linda Thorson as Tara King
Peter Barkworth as Jimmy Merlin
Joss Ackland as Brigadier Hansing
Brian Blessed as Sergeant Hearn
Penelope Horner as Jenny Firston
Donald Douglas as Major Parsons
Philip Dunbar as Yates
Jonathan Scott as Cartney
Nosher Powell as Ministry Guard
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The towns of St. Albans, Watford and Old Hatfield were used as to represent the unnamed town in "The Morning After", the locations in which remain virtually unchanged since filming, with the exception of Old Hatfield's central area.