Scotland Yard detectives attempt to solve a spate of safe robberies across England beginning with clues found at the latest burglary in London. The film is notable for using a police procedural style made popular by Ealing in their 1950 film The Blue Lamp. It is known in the US as The Third Key.

Jack Hawkinsas Supt Tom Halliday
John Strattonas Sergeant Ward
Dorothy Alisonas Mary Halliday
Sam Kyddas Police Constable in Information Room
Glyn Houstonas Detective Sergeant in 'Q' car
Richard Leechas Nightwatchman / Gilson
Geoffrey Keenas Chief Superintendent Jim Malcolm
Sydney Tafleras Stone