Suave and urban Paul Temple (John Bentley) is a former Scotland Yard investigator who has become a successful mystery novelist. Paul frequently lends a hand in the solution of real-life crimes that challenge even the most experienced Scotland Yard sleuths. At his side in these exploits is Steve (Dinah Sheridan), his lovely wife and partner-in-crime-solving. As the story begins, Scotland Yard is unable to unravel the "Rex" murders. An unidentified killer who signs his name as "Rex" has slain three wealthy women as they rode in trains. All of the victims carried pieces of paper on which was written the name "Mrs. Trevellyan".