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?

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.

Iris Archer agrees to appear in one of Paul Temple's plays, but then disappears until he sees her in a Scottish hotel when she is supposed to be in France. But how is this connected with a letter passed to Temple with a plea to deliver it to a Mr Richmond, and a network of spies with a shadowy leader called Z4?