The Morning After The Avengers : Season 7 Episode 18

Having been rendered unconscious by knock-out gas in his pursuit of rogue agent Jimmy Merlin, Steed awakes to find that London is deserted and under martial law, headed by Brigadier Hansing. Under cover of a supposed evacuation, the Brigadier, incensed that he is to be retired from the army, has planted an atom bomb to destroy the capital. All Steed has to do is find it.