Notorious criminal Merlin breaks into a research lab and steals some nerve gas capsules, but doesn't realise his contact is in fact Steed, hell-bent on capturing him. Attempting to escape, Merlin smashes a capsule, and they all fall asleep for a day...
Steed wakes, and handcuffs himself to Merlin while Tara slumbers on obliviously. They emerge into an eerily deserted town, cars abandoned, bank doors ajar and money fluttering down the street. something's wrong, very wrong.
They soon discover Cartney, recently shot by a military firing squad and discover that martial law has been imposed and the population evacuated after an atom bomb alert; a squad is currently trying to defuse the bomb. Merlin recognises one of the officers as a foreign agent, and they discover that the 'soldiers' are in fact installing a bomb.
Merlin recovers more of his gas capsules, and they knock out the mercenaries and their leader, the turncoat Brigadier Hansing.
Back at Steed's apartment, Merlin has sent a present, an apparently empty box; they discover what it contains when a phone call informs Steed that Merlin has stolen some luminous dust.

Produced: 5 Nov 68
Broadcast:
London: 19 Feb 69
Midlands: 13 Jul 69
New York: ?