Swinging between London and Geneva, Paul Temple and Steve are involved in a case that involves finding the answer to one vital question: is Carl Milbourne, thought to have perished in a car accident, still alive? There are those who are determined to stop the Temples discovering the truth. And Temple's own life is in danger.

Returning from America by ocean liner, the Temples meet wealthy Sam Portland whose UK agent Hubert Greene has received messages about Portland's past from a private detective called Madison. But Portland dies on board, and Temple is left to unravel a case involving international counterfeiters and the clue of a coin on a watch-chain.

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?

After a series of robberies, Scotland Yard are mystified. In the latest a night watchman is attacked, and in his last breath manages to gasp out 'The Green Finger'. This is not the first time the phrase has come to the attention of the Yard; it was also said by a man who was wanted in connection with another robbery as he died from drowning. Who or what is the 'Green Finger'? The public are alarmed and the press demand that the Yard 'Send for Paul Temple' Aided by a young, pretty, reporter known as Steve Trent, Paul is faced with solving a deepening and widening mystery.

Sir Graham Forbes of Scotland Yard often calls upon Paul Temple to help with his latest unfathomable case...