[5] In preparing for a secret raid on a German-held French coastal village, a British security officer is chosen to monitor activities in England among army personnel of the 95th Infantry as well as civilians with whom they mingle.

At the same time, German intelligence send Agents 23 and 16 to England to obtain information from sources including conversations overheard in pubs, railway stations, shops and other public places.

Agent 16 is caught, but 23 reaches his contact, Mr Barratt, a bookseller at Westport, who assigns him the job of infiltrating an ordnance depot.

An agent manages to steal the briefcase containing an aerial negative, carelessly left unattended at a cafe by a wing commander.

[6] On the advice of film mogul David O. Selznick, who believed that American audiences might get the impression that Britain was overrun with spies,[7] the director, Thorold Dickinson, made drastic cuts to the running time, but also added two short extracts from a speech by J. Edgar Hoover as a framing device.