The movie begins with the arrival of a group of characters at a remote cottage in Scotland, owned by Mrs. Barrington (Jeanne De Casalis). Among the guests are an inventor named John Barrington (George Cole), a British secret agent named Charles Dimble (Leslie Banks), and a couple named Richard and Carol Bennett (Alastair Sim and Carla Lehmann).
As the guests settle in, it becomes apparent that one of them is a Nazi spy who is after Barrington's invention, a new type of bombsight that could potentially change the outcome of the war. The spy, posing as a handyman, attempts to kidnap Barrington but is thwarted by Dimble.
The tension escalates as more attempts are made to steal the bombsight, leading to a dramatic climax in which the spy is revealed and the guests band together to protect Barrington and his invention.
The film is notable for its witty dialogue and comedic moments, particularly from Alastair Sim's character. It was a box office success and helped launch the career of John Mills, who played a small but memorable role as a young soldier named Robbie.