A wave of mysterious robberies is sweeping the country, and fifty thousand pounds’ worth of diamonds has been stolen in six months.

During the fourth raid, a nightwatchman is attacked with chloroform, and just manages to gasp out ‘The Green Finger’ before he dies.

Chief Inspector Dale has heard the enigmatic phrase before: it was the last utterance of a drowning man wanted in connection with another robbery.

But with no further leads, the police are baffled. The cry goes out in the popular press: ‘Send for Paul Temple!’