In the early 1950s, the British Prime Minister (Ronald Adam) is sent a letter by Professor Willingdon (Barry Jones), who works at Britain's atomic weapons development facility, the (fictitious) Wallingford Research Centre, from which he has surreptitiously taken a nuclear warhead. It is a very explicit threat that Willingdon will destroy the centre of London in a week's time, at noon (hence the film title), unless the British government declares that it is to stop all stockpiling of nuclear warheads. Detective Superintendent Folland (André Morell) of Scotland Yard's Special Branch is charged with tracking down Willingdon and stopping him.

Arriving at the Wallingford Research Centre (based on the Atomic Weapons Research Establishment {AWRE} at Aldermaston), Folland's team find Willingdon missing, along with a nuclear bomb. Willingdon's assistant Lane (Hugh Cross) is recruited to help and they return to London to search for him.

Willingdon, carrying his bomb in a Gladstone bag, finds lodgings with Mrs Peckitt (Joan Hickson), but spooks her with his constant pacing around his room during the night. The following morning, he leaves early and, seeing a 'wanted' poster with his face, disguises himself with a new coat and having his moustache shaved off.

Folland's team plan for the worst and get Cabinet approval to evacuate London. Rumours begin to fly that another war is about to be declared, and the Prime Minister agrees to do a radio broadcast to try to quash these, and appeal to Willingdon to give himself up.

The next day, Willingdon's daughter Ann (Sheila Manahan) turns up at Folland's office to demand some answers. Folland tells her all, and asks her to stay and help – she may be the only person the professor will listen to.

Mrs Peckitt reports Willingdon to the police, thinking that he is a 'landlady murderer' reported in the paper, but a quick-thinking constable realises the description better matches Willingdon and a car is sent to check him.

Unfortunately, Willingdon spots it on his way back to his lodgings and makes a quiet getaway. Driving back to their hotel from the police operations centre, Lane and Ann Willingdon spot the professor but fail to catch him. An updated description is quickly circulated.

That evening Willingdon bumps into 'Mrs' "Goldie" Phillips (Olive Sloane); she invites him to buy her a drink, the two of them having met, by chance, earlier at a pawnbroker's. As he has no lodgings, Goldie offers him her "spare" bed for the night. By this time, London is being evacuated and Willingdon decides to lie low. The troops have begun to search and Goldie's bedsit seems a good place to remain hidden. Willingdon is forced to hold Goldie hostage, fearing that if he doesn't, she will inform the authorities of his location.

The streets cleared, Willingdon makes his escape and finds his final refuge, a bomb blitzed church. The net steadily closes and Willingdon is finally found, praying. Lane, Ann and Folland arrive to try to talk the professor away from his bag. He panics, runs from the church, and is killed by an even more panicking soldier (Victor Maddern). With seconds to spare, Lane has the bomb defused.
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== osoby ==			== profese ==
	John Boulting		Režie
	Roy Boulting		Režie
	Roy Boulting		Scénarista
	John Boulting		Střih
	John Boulting		Producent
	Roy Boulting		Producent

cena	festival	ročník	osoba	nominace/cena
Zlatý lev za nejlepší film	Benátský filmový festival	15 (1950)	John Boulting	nominace
Zlatý lev za nejlepší film	Benátský filmový festival	15 (1950)	Roy Boulting	nominace
Nejlepší britský film	BAFTA	4 (1950)	John Boulting	nominace
Nejlepší britský film	BAFTA	4 (1950)	Roy Boulting	nominace