The Third Key

Scotland Yard detectives Supt Tom Halliday (Jack Hawkins) and Detective Sergeant Ward (John Stratton) investigate a series of safe robberies across England, following clues found at each burglary. They deduce that the thief is being tipped off by an insurance agent about which safes contain significant amounts of cash. The investigation leads to Mrs Gilson, who fakes her own death after spending years making duplicate keys for all the safes her company produced.