The platoon is given the job of signposting the area for an Army exercise, but a steam engine is blocking the route. When Jones tries to drive round it, his van becomes stuck, blocking both Hodges in his van and Mr Farthing with his bus of pensioners.

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.