Actress Norma Rice is found dead on a train, and the letters REX are scrawled in red chalk on her compartment window. It is the third death to occur in a mysterious string of murders and Scotland Yard are compelled to send once again for Paul Temple. Temple, now acting as an investigator as well as a mystery novelist, is joined by his wife Steve as they are embroiled in this latest mystery. As they convene to discuss the case with 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 Steve and Paul unmask 'Rex' before they strike again?