In spring 1944, the tension rises as German forces actively seek intelligence regarding the imminent Allied invasion. Their efforts hit a snag when MI5 agents, Major Cunningham and Captain Meeks, intercept one of their spies in London. Across the English Channel, in the coastal town of Walmington-on-Sea, Captain Mainwaring finds his Home Guard unit struggling for recognition of their contributions to the war effort.
The situation takes a turn when the platoon, during a routine exercise, inadvertently forces Rose Winters, an elegant journalist, off the road. This incident sparks excitement in the platoon as they learn she plans to write an article about them for a national magazine. However, her presence also ignites rivalry among the townsfolk, particularly with their wives. Adding another layer of complexity, Sergeant Wilson realizes that he had once tutored Rose during her time at the University of Oxford, which leads to a budding friendship.
Unbeknownst to the platoon, Rose is, in fact, a spy sent by Germany to gather crucial information about the Allied invasion plans. Her activities do not go unnoticed, as the transmission she makes via radio is swiftly intercepted by Major Cunningham and Captain Meeks, prompting them to investigate further. Upon meeting with Mainwaring and briefing him on the delicate situation, he sees an opportunity to protect his platoon from disbandment by their senior officer, Colonel Theakes, after a mishap during a patrol near Dover.
Seizing the chance, Rose joins the next patrol through Private Joe Walker, who inadvertently reveals his illegal bootlegging business. During this outing, Rose cleverly extracts vital information regarding the Allies' strategy, including the specific location of the upcoming invasion. Under Mainwaring’s command, the platoon is tasked with uncovering the identity of the spy among them. However, Rose skillfully misleads Mainwaring into believing that it is Wilson who is the traitor, leading to his arrest.
The narrative unfolds further when Mainwaring decides to meet with Rose to thank her for her perceived help. Meanwhile, the rest of the platoon detains Wilson for more questioning. However, before long, evidence surfaces that exposes Rose as the genuine spy they have been searching for. The tension escalates when they finally track her down, discovering her holding Mainwaring captive alongside a Wehrmacht landing party. He had stumbled upon the truth after witnessing a German U-boat emerging in a nearby bay.
The Home Guard, with assistance from their wives, gallantly intervenes to rescue Mainwaring, managing to capture Rose and the landing party, which forces the U-boat to retreat. In a swift turn of events, Cunningham and Meeks arrest Rose, ensuring that her intelligence remains secured and does not reach enemy hands. Colonel Theakes recognizes the platoon's efforts, saving them from disbandment. The story concludes with Mainwaring and Wilson mending their friendship, as both men join their platoon in a celebratory parade through Walmington-on-Sea, embracing their hard-won victory and renewed camaraderie.
