Third and final film in the 'Inspector Hornleigh’ series of comedy-thrillers. Inspector Hornleigh (Gordon Harker), disappointed at not being handed an important spy case, is assigned by Scotland Yard to an army barracks to investigate the mundane thefts of supplies from the stores. This accidentally leads Hornleigh and Sergeant Bingham (Alastair Sim) to a nest of fifth columnists when his dim-witted assistant carelessly talks to a girl in the cafeteria – and that night, news of Hornleigh and Bingham’s arrival is embarrassingly transmitted back to Germany.