John Barrington (Leslie Banks) is a talented inventor, currently working on a bombsight for the Royal Air Force. who prefers to work in his own country house in the Highlands of Scotland near Loch Tay. His eccentric wife (Jeanne de Casalis) has agreed to take in child evacuees from London to be accommodated in a nearby cottage they own. However, also arriving is Charles Dimble (Alastair Sim) to whom the cottage has been let by the letting agency. To compound the confusion, Mrs. Barrington had also agreed to allow it to be converted into a military hospital. Therefore, Mrs Barrington decides she can only take one evacuee, which turns out to be cocky teenager, Ronald (George Cole).

An injured pilot who has parachuted into the nearby loch is rescued and brought to the house, becoming the first patient for the new hospital. Mrs. Barrington moves Ronald to the main house, while Dimble and the pilot remain in the cottage. After his injuries are treated by Mrs. Barrington's daughter Helen (Carla Lehmann), the pilot identifies himself as Flight Lieutenant Perry, flying Spitfires from a nearby airfield. However, when he is given a telephone to call his base, he makes the call, overheard by Helen, with the telephone disconnected from the socket.

The War Office discuss Barrington, concerned that someone is spying on his work, since his last invention, a self-sealing fuel tank, was copied by the Germans within a month of its mass production. They suspect his assistant Alan Trently (Michael Wilding), who although British was educated in Germany and still corresponds with people in Switzerland, and it is mentioned that they have sent someone to the house to investigate.

Defying the house rule, Ronald goes into the laboratory. He overcomes Barrington's initial hostility with his practical knowhow and the two become friendly. Trently becomes jealous when Helen starts spending time with Perry. However, Helen resists Perry's advances and eventually lets Trently know that she prefers him.

German agents make their move and kidnap Barrington. Ronald stows away in the car used to take the captive to an isolated water mill. When Perry shows up, Ronald attacks one of the spies to help in the "rescue", but is shocked when Perry is revealed to be the agents' ringleader and intends to take Barrington to Berlin on a seaplane which is due to arrive in the loch the next night.

It turns out that Dimble is the British counterintelligence officer sent by the War Office. He manages to infiltrate the spy ring and learn where Barrington is being held. All but one of the spies are captured and the prisoners are freed. Perry initially escapes, but is eventually tracked down and killed in a shootout with Dimble. (fonte: Wikipedia)