U.S. Treasury Investigator Joe Dennison is sent to England on the trail of a gang of thieves responsible for stealing a million dollars from a U.S. Treasury vault. One of the crooks is on his way to England to spend the money on phony diamonds. Scotland Yard Inspector McClaren, assigned to work with Dennison on the case, is also working on the disappearance of Dr. Eric Miller, an atomic scientist. Dennison discovers while interviewing Miller's daughter Marlene that she is wearing a brooch containing a phony diamond, and the brooch had been made by her father.