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.