[3] In the spring of 1944, German forces work to secure intelligence about the upcoming Allied invasion, after one of their spies is intercepted in London by MI5 agents, Major Cunningham and Captain Meeks.

Unknown to the platoon, Rose is actually the spy sent by Germany to investigate the Allied invasion plans, but a transmission she sends by radio is detected by Cunningham and Meeks, who travel there to seek her out.

After meeting with Mainwaring and informing him of the situation, he takes delight in using this as a means to save his platoon from being disbanded by the Home Guard's senior officer, Colonel Theakes, after a disaster during an important patrol near Dover.

Tracking her down, they quickly find her holding Mainwaring prisoner with a Wehrmacht landing party, after he discovered the truth upon seeing a German U-boat appear in a nearby bay.

Ian Lavender makes a cameo as Brigadier Pritchard, providing a link with the original series,[4] and Frank Williams reprised his role as the vicar.

Mrs. Mainwaring, who was a completely unseen character for the whole of the original series, now has a prominently visible role in the film, where she is portrayed as a chief volunteer of the local Auxiliary Territorial Service, and is even more pompous, domineering and vociferous than her husband.

"[12] Peter Bradshaw of The Guardian was less convinced, giving it two stars, stating that "it's hard to escape the sinking feeling that this is a waste of talent – that this is a good natured, well meaning but pointless kind of Brit comedy ancestor worship; paying elaborate homage to a TV show that got it right the first time.