When six members of the Highland Regiment attack the Warmington-on-Sea's HQ, the platoon has to come up with a scheme to capture the attackers.

When the church bells start to ring, the platoon leaps into action. Jones takes half the platoon to Godfrey's cottage to set up a machine gun post while Wilson, not knowing where Jones has gone, also heads for the cottage where they both mistake each other for the enemy and prepare to attack.

Walker is finally called up but Mainwaring tries to get his call-up deferred because of his value to the platoon, but he is still conscripted, only to find out that he is allergic to the army's corned beef.

Sgt. Wilson overhears Mrs. Pike telling Frank that she is expecting a child. Wilson assumes that she is pregnant and that he is the father, but it turns out that she is looking after an evacuee. Mainwaring advises that Wilson marry her, until she clears up the misunderstanding.

Capt. Mainwaring promotes Frazer to Lance Corporal, thus causing friction between him and Jones. When Frazer starts getting above himself, Mainwaring cancels the promotion.

The platoon is fire watching when the men see a man using a torch. Convinced that they have discovered a spy, they arrest him only to find that they have made a terrible mistake.