May 1940: the 'phoney' war is about to come to an end. The head of the German Secret Service is attempting to flood the UK with spies and Dalby and Mackenzie are determined to uncover them. On May 10th France is invaded and the British army retreats to Dunkirk. In a race against time, Dalby and Mackenzie hatch a daring plan: they will destroy the brothel in Dunkirk from which the 'deadly ladies' are doing their spying on British troop movements, and they will capture the 'pianist' - the unknown German radio operator who is reporting back from Dover Castle. They must find him and stall the last German attack on Dunkirk so that the army can escape at last.