When a landlady finds one of her tenants murdered, Inspector Hornleigh is sent to investigate. Inspector Hornleigh's assistant, Sergeant Bingham, soon finds an attaché case that had been stolen from the murdered man. When Hornleigh examines the case, inside it he finds a bag that was used to carry important government documents. The documents have been taken, and to make things even more confusing, a duplicate of the stolen bag soon turns up.

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.

During a holiday by the British seaside, Hornleigh and Bingham grow bored and turn their hand to investigating a local crime.