S3·E6 Room at the Bottom

Wilson secretly enjoys passing on HQ Captain Bailey's finding that Mainwaring's captain's commission in the original local militia days is invalid, so he'll be reduced to lieutenant, as befits a platoon commander. When HQ concludes he's actually not even an NCO, he has to join the ranks under the Guard drill instructor he ordered himself. After the platoon's embarrassing failure at an exercise, everyone wants to have him reinstated, except Frazer, who smells opportunity.