Whinfrey's Last Case 1979

In 1913 the British government, hearing that the Germans are planning World War I, ask top agent Whinfrey to intervene, but instead he decides to go on holiday to Cornish fishing village Torpoint. Here he believes he has stumbled upon a smuggling ring, but in fact they are Germans who are masquerading as locals in an advance invasion party. The British arrive and arrest them, believing that Whinfrey deliberately planned everything - though it is apparent that he did not.