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.