John Barrington, a talented but eccentric inventor, toils at his Scottish country home on a sophisticated bombsight for the RAF. Leslie Banks His scatter-brained wife, Jeanne De Casalis Mrs. Barrington, welcomes London evacuees to stay in a nearby cottage they own, hoping to do their bit for the war effort. Yet with the estate already letting the cottage to Alastair Sim Charles Dimble, Mrs. Barrington decides they can take only one evacuee—the cocky teenage boy named George Cole Ronald. George Cole The cottage briefly becomes a hybrid home and makeshift clinic, as Barrington’s daughter, Carla Lehmann Helen Barrington, steps in as a nurse for the wounded.
An injured pilot parachutes into a nearby loch and is rescued, his identity soon confirmed as Flight Lieutenant Perry, stationed at a nearby airfield. John Mills Flight Lieutenant Perry arrives at the house, where the family and evacuees are still adjusting to the wartime routine. Yet when Perry is given a telephone to contact headquarters, he speaks to the office alone and uncouples the call by disconnecting the wire, a small but telling sign of the tensions surrounding Barrington’s work. The War Office grows wary, quietly suspecting that someone nearby might be spying on Barrington’s top-secret bombsight project, and they zero in on his assistant, Michael Wilding Alan Trently, who was educated in Germany and maintains correspondence with people in Switzerland.
Back at the estate, the atmosphere thickens. Ronald begins to push boundaries by wandering into Barrington’s laboratory, and his practical, fearless outlook gradually erodes Barrington’s initial hostility. The two form an unlikely bond as the war’s pressures press in from every side. Meanwhile, Helen finds herself drawn toward Perry, even as she tries to balance loyalty and caution, and she makes it clear to Trently that her affections lie elsewhere. The delicate love triangle adds a human counterpoint to the escalating espionage heat.
One night, Barrington is abducted by German agents. Ronald, ever resourceful, hides in the car’s boot and follows the captors to an isolated water mill where Barrington is kept. Ronald slips inside and soon realizes Perry’s true role: he is not a wartime ally but a German intelligence agent plotting to fly Barrington to Berlin by seaplane. The plot thickens as Dimble, the man who initially let the cottage, is revealed to be a British counter-intelligence officer sent to dismantle Perry’s spy ring. He infiltrates the group, locates Barrington’s location, and coordinates the counter-plot from within.
The net tightens as Dimble and his associates close in on the remaining spies. Ronald and Barrington are freed as the last of the conspirators are captured or neutralized. Perry makes a desperate bid to escape but is ultimately tracked down in a tense, tense shoot-out with Dimble. The film closes on the uneasy balance between scientific ingenuity, personal loyalties, and the high-stakes chess game of wartime espionage, where a quiet Scottish estate becomes a front line against covert threats and the everyday heroism of ordinary people thrust into extraordinary danger.
