Private Walker, source of the platoon's supplies of whisky, cigarettes and other essential black-market goods, is called up to serve in the regular Army, so Mainwaring decides to use all his standing as an officer and a bank manager to have him excused from military service. Meanwhile, Jones and the others try to ensure that Walker fails his medical.
When he's told he can make a member of the platoon a corporal, Mainwaring instigates a competition between Frazer and Jones to see who deserves the promotion. Both do their best to impress him, particularly during a demonstration on how to face down a Nazi stormtrooper, but things go awry when a power-crazed Frazer charges the other men with court-martial offences.
Tipped off by Frazer and Godfrey, Mainwaring and Wilson investigate a house from where they believe someone may have been signalling to enemy aircraft. They take the suspect back to the church hall, but chaos ensues when an incendiary bomb comes through the roof and the platoon desperately battles the blaze.