In 1950, on a Monday, the British Prime Minister Ronald Adam receives a chilling message: a man claims to have already stolen a nuclear weapon and pledges to level the center of London next Sunday at noon unless the government renounces its development of such devices. The letter is signed “Professor Willingdon,” Barry Jones Willingdon, the senior researcher at Britain’s fictitious Wallingford Research Centre. Detective Superintendent Folland of Scotland Yard’s Special Branch, André Morell, is assigned to decide whether this is a calculated hoax or a genuine, imminent threat.
At the Research Centre, Folland discovers that Willingdon has vanished, along with a UR12 bomb—a compact device small enough for a single person to carry. He recruits Willingdon’s assistant, Stephen Lane Hugh Cross, to assist in the search, and the pair head to Willingdon’s home. There they learn that neither Lane nor Willingdon’s wife or daughter, Ann Willingdon Sheila Manahan, noticed anything out of the ordinary, but troubling notes tucked into Willingdon’s papers, coupled with remarks he made to the local vicar—the last person known to have spoken with him—suggest he came to believe the government was misusing his work for evil.
On Tuesday, Willingdon, still lugging the bomb around in a Gladstone bag, spots his photo in the papers and promptly shaves off his moustache before seeking shelter. He rents a room from Mrs. Peckett Joan Hickson, but his late-night pacing unsettles her. After he leaves, she reads a report about a wave of disappearances of landladies and immediately calls the police. A quick-thinking constable Joss Ackland realizes the description matches Willingdon, and a patrol car is sent to the boarding house. Willingdon returns briefly, sees the police outside, and manages a quiet escape.
Willingdon discards his overcoat and buys another at a pawn shop, where he runs into Goldie, an actress whose best days are behind her, and her dog, Trixie. The trio are drawn back together when Trixie leads Goldie to Willingdon’s whereabouts. They end up at a pub, and when it closes, Goldie invites Willingdon to stay at her apartment. He sleeps on a spare bed and leaves before she wakes the next morning.
Meanwhile, the press picks up on the frantic Cabinet meetings and the sense of impending mobilization. Rumors of war swirl as the Prime Minister prepares to speak to the nation by radio. He announces an evacuation to begin the next morning for a 12 square miles around Parliament, after which a centralized London-wide search will commence from the outskirts toward the heart of the city.
Goldie spots posters bearing Willingdon’s image and goes to the police. When she gets home to pack for the evacuation, she finds Willingdon waiting—nervous and focused—holding her hostage and threatening to detonate the bomb prematurely if she calls for help.
The evacuation proceeds smoothly, and the military begins its painstaking sweep of central London. The operation is aided by Army units and specialists on the ground, including an Army Mechanic John Stratton. As Saturday gives way to Sunday, the net tightens around Goldie’s street, but Willingdon makes a final break for freedom.
Shortly before noon on Sunday, Willingdon is found praying in a church badly damaged during The Blitz. Folland and Lane rush to the scene, bringing Ann Willingdon Sheila Manahan in an effort to calm him. He insists it’s too late to change his plan, and Lane begins to defuse the UR12 bomb. Willingdon, shouting that it is “Too late,” breaks free, runs from the church, and is shot by a nervous soldier as the clock nears twelve. Lane completes the disarming just in time, and the all-clear sirens sound. On Westminster Bridge, Goldie, who had gone to hitch a ride to Aldershot, hears the signal and heads home, relieved that the danger has passed.
