Set during the Second World War, the film recounts the adventures of William Potts (Will Hay) after it is discovered that he is an exact double of a German spy who the British have just captured.

Potts is flown into Nazi Germany to impersonate the spy and instructed to seek out and bring back details of a new German secret weapon.

Potts attempts to undermine this by convincing the youngsters that the proper British way of saluting a great leader is to apply the V-sign, which they therefore do repeatedly and enthusiastically in the direction of a portrait of the Führer.

At a function where he hopes to gather information about the weapon (a gasfire bomb), Potts succeeds only in getting blind drunk and admitting that he is a British agent.

Potts and his new friends eventually commandeer a plane and fly back to Britain, crashing in a tree outside the War Office in London.