Ewen Montagu and chronicles Operation Mincemeat, a 1943 British intelligence plan to deceive the Axis powers into thinking the Allied invasion of Sicily would take place elsewhere in the Mediterranean.

[2] In 1943, Royal Navy Lieutenant Commander Ewen Montagu (Clifton Webb) devises a scheme to deceive the Nazis about the impending invasion of Southern Europe.

The non-existent Royal Marine courier, Major William Martin, would appear to be a plane crash victim carrying documents about an upcoming Allied invasion of German-occupied Greece, rather than Sicily, the more obvious target.

Following a medical expert's advice, Montagu obtains the permission of the deceased's father and procures the body of a man who died of pneumonia, which would give the appearance he had drowned.

After the attaché case containing the deceptive documents is returned to London, a forensics expert confirms that the key letter, which describes an Allied invasion of Greece, has been cleverly opened, photographed and resealed.

O'Reilly investigates Martin's American "fiancée", Lucy Sherwood (Gloria Grahame), who is the flatmate of Montagu's assistant, Pam (Josephine Griffin).

Attached to the dead body was a briefcase containing fake letters falsely stating that the Allied attack would be against Sardinia and Greece rather than Sicily, the actual point of invasion.

On the one hand, certain accounts claim the true identity of "Major William Martin" was a homeless, alcoholic rat-catcher from Aberbargoed, Wales, Glyndwr Michael, who had died by self-administering a small dose of rat poison.

However, in 2002, authors John and Noreen Steele published the non-fictional account of The Secrets of HMS Dasher, about an ill-fated escort carrier that exploded and sank in the Firth of Clyde around the time Operation Mincemeat had commenced.

To support his claims, Smyth published the contents of a secret memo and an official report, both authored by Ewen Montagu, confirming the Glyndwr Michael story.

Written by Spike Milligan and Larry Stephens, the first version of the script formed two-thirds of the episode broadcast on 31 March 1953, before the film's release, with the first third comprising a separate sketch.