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.

A young girl is travelling to London to find work. Arriving at the station, she meets a man who has been stabbed by a member of a gang of crooks involved with greyhound racing. She becomes a suspect, but flees the scene in order to deliver a message to the dead man's brother. She is protected from the police by a night club entertainer, who she learns is the man she is seeking.

The mirthful adventures of Police-Sergeant Samuel Dudfoot and his two constables, Albert Brown and Jeremias Harbottle, who stage a fabricated crime-wave to save their jobs---and then find themselves involved in the real thing.