The platoon participates in an invasion exercise with Captain Ogilvie and a Highland regiment.
The platoon mistakenly believes an invasion is in progress, and splits into two sections. One section, though, mistakes the other for the enemy.
Walker is called up into the army. The platoon, anxious at how they will obtain off-the-ration supplies without him, fight to keep him.
Private Pike's mother agrees to take in a child evacuee, but Wilson misunderstands her and believes she is pregnant. Mainwaring orders Wilson to marry her.
Mainwaring has the opportunity to promote someone to corporal. Rather than promote Jones, he tests who has the greatest potential by temporarily promoting Private Frazer to lance corporal. Frazer's increasingly dictatorial manner soon alienates the platoon.
When Frazer spots what he believes to be a German spy signalling planes, the platoon arrests a suspect who protests that he is a naturalised Englishman.
