The big war news of the day is the unexpected arrival of a German officer who parachuted down in Scotland. Colonel Witherton is given the task of organising the officer's transport to the Tower of London, a project known as Mission Dirty Washing. He has some other things on his mind, though. General De Gaulle needs to get new quarters (Operation Frere Jacques) and there's a regimental dinner the next night. The colonel orders Potts to clean his dress uniform and look after the regiment's mascot, a nanny goat.
While visiting the colonel's offices, Potts loses sight of the goat for a moment and it starts to nibble at some documents. Sally Perks and Potts discover that only the envelopes of two important documents were damaged and so they type up new envelopes for Mission Dirty Washing and Operation Frere Jacques. They finish just in time before the French general's aide, Major Henri Boucher, walks in to collect his instructions for De Gaulle's move.
Shortly afterwards Colonel Witherton receives news that the German prisoner of war is very happy with his suite at Harridges. De Gaulle, on the other hand, takes his move to the Tower of London as an insult. Major Henri Boucher drops by to challenge Witherton to a duel. When Potts admits that he is responsible for the mix-up, Boucher decides that Potts should fight with a man of his own rank, Boucher's servant Michel.
The next morning Witherton, McRuckus and Potts meet Boucher and Michel at St James's Park. As the English were allowed to choose a weapon, Witherton opted for guns and put blank cartridges in both guns. Unfortunately the French bring their own guns. The duel in the very foggy conditions eventually leaves only one victim: Michel shoots a duck which Potts serves for dinner later that day.