There is some tension at the British Government underground headquarters as it is suspected that an enemy agent is leaking information. Private Potts overhears two English officers talking German and immediately notifies Colonel Witherton. Witherton ignores Potts' warning and puts his trust in a special powder created by Professor Jenkins. The powder will be left on a confidential document. Whoever handles the document, will get green fingers.
In the corrider Jenkins accosts Potts, asking him to hand out leaflets from the Ministery of Information. In the batmen's room Potts listens to the radio. When he accidentally switches on the German propaganda programme he hears Lord Haw-Haw betray information that Potts read earlier that day in Churchill's office. He is now sure that someone at the headquarters is leaking information.
Potts rushes to Witherton with the news. As they are talking, they hear the two officers talking German again. Witherton has them arrested but soon discovers that they are being trained to monitor German broadcasts. The situation becomes completely embarrassing for Potts when Witherton notices green dye on the private's finger.
In the brig Potts moans about his situation but he is surprised to be joined by Colonel Witherton, who also has green dye on his hands. The two men realise that the powder must have come from the leaflets that they both handled. This means that Professor Jenkin must be the leak.
They are released from prison and confront Jenkins in the war room. Jenkins takes Potts hostage, but Potts can disarm him easily. By shouting 'Heil Hitler' he makes Jenkins lift his right arm, pointing the gun away from him. Potts' moment of glory does not last very long: his trousers fall down. (In prison they removed his braces and belt.)