A woman is found dead on a train, and the name "Rex" is written - this is the third in a mysterious string of murders and Scotland Yard are baffled. They send for Paul Temple again. An investigator, who is now a mystery novelist, Temple and his wife Steve become involved in this latest mystery. Meeting the Yard's Sir Graham Forbes at a nightclub, they witness one of the performers die in the middle of her act before they have a chance to speak to her. Can Temple unmask 'Rex' before he (or she) strikes again?