Government officials discover a horrible plot: the Chinese are tunneling their way to the United States.

A slick young man buys a jewel with a cheque that bounces. He then disappears and both the police and a lawyer try to find him. The lawyer hires a female detective to track down the missing man, but when his body is found in the Thames the case is by no means over.

An escaped murderer flees France to England, where he forces an ex-forger, now established as a reputable estate owner, and the forger's daughter who knew nothing of his past, to counterfeit 5-pound notes for mass distribution around the countryside.

A man pays a hitman to kill him. Circumstances change and he tries to call off the hit but he has trouble getting the deal killed.

When his wife stops writing to him and his letters are returned unanswered, Dan becomes extremely concerned about her welfare. He returns home but is only able to meet with her briefly before she is found murdered. Dan is the obvious suspect but has only 36 hours to find out who murdered her. In so doing he uncovers a shocking catalogue of his wife's past affairs and an identity that he knew nothing about.

While in London trying to nab several fugitives who stole a million dollars from a U.S. Treasury vault, American federal agent Dennison (Dennis O'Keefe) assists Scotland Yard Insp. McClaren (Philip Friend) in his search for a missing atomic scientist (Paul Hardtmuth). The detective duo discovers that the key to solving both cases may rest with the scientist's daughter (Margaret Sheridan) and her fascinating diamond.

Tom Masterick, a dock worker, is wrongfully convicted of a murder charge. His death sentence is commuted to a long prison term. When released as an old man, he vows to find the real killer.