On a field exercise acting as commandos meeting a secret agent (represented by Mainwaring), the platoon must trust no one for GHQ has sent out counter agents to catch them. The appearance of a gorilla compounds their problem.

Lady Maltby donates her Rolls-Royce for use as a staff car. Wilson and Pike are assigned to paint it for camouflage, but instead they mistakenly paint the Mayor's Rolls-Royce, just before a French general is due to visit the town.