The story is about a long-established and highly respected family firm of auctioneers, the House of Caradus, in Chester, England, which is now in serious financial trouble. The series captures the drama of the auction room, the excitement of bid and counter-bid, taking in dealers' rings, forged art treasures and the growing invasion of the antiques scene by London-based auction houses starved of pieces to sell. As the House of Caradus struggles to stay alive, by fair means or foul, its clients cover the whole spectrum of need and greed.

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.