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. Meanwhile, Captain Mainwaring finds his Home Guard in the coastal town of Walmington-on-Sea suffer from a lack of appreciation for their work in contributing to the war effort. Things change when the platoon, during an exercise, accidentally force Rose Winters, an elegant journalist, off the road. When they learn she intends to write a report about them for a national magazine, the men feel delighted, but also are charmed by her presences, causing them to have feuds with the townsfolk, and especially with their wives. The matter is further complicated when Sergeant Wilson develops a friendship with Winters after recognising as a student he once tutored at the University of Oxford.
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. Rose swiftly joins the patrol through Private Joe Walker, after discovering his illegal bootlegging business, and uses the next patrol to discover important details on the Allies' plan, including where the invasion will take place. As the platoon work to find the spy on Mainwaring's orders, Rose deceives him in believing the spy is Wilson, who is promptly arrested.
While Mainwaring decides to meet with Rose to thank her for her assistance, the rest of the platoon detain Wilson for questioning, until evidence is brought to their attention that Rose is the spy they were seeking for. 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. However, the platoon help to rescue him with the aid of their wives, and manage to capture the landing party and Rose, causing the U-boat to flee. Cunningham and Meeks swiftly arrest her, ensuring her intelligence doesn't return, while Theakes commends the platoon, saving them from being disbanded. Mainwaring and Wilson reconcile, and both men join their platoon for a parade through Walmington-on-Sea.
Toby Jones as Captain Mainwaring
Bill Nighy as Sergeant Wilson
Catherine Zeta-Jones as Rose Winters
Tom Courtenay as Lance-Corporal Jones
Blake Harrison as Private Pike
Michael Gambon as Private Godfrey
Bill Paterson as Private Frazer
Daniel Mays as Private Walker
Sarah Lancashire as Mrs Pike
Mark Gatiss as Colonel Theakes
Mark Tandy as Major Cunningham, MI5
Andrew Havill as Captain Meeks, MI5
Emily Atack as Daphne
Alison Steadman as Mrs Fox
Holli Dempsey as Vera Shilton
Annette Crosbie as Cissy Godfrey
Ian Lavender as Brigadier Pritchard
Frank Williams as the Reverend Timothy Farthing (Credited only as 'The Vicar')
Felicity Montagu as Mrs Mainwaring
Martin Savage as Warden Hodges
Oliver Tobias as Admiral Canaris
Julia Foster as Dolly Godfrey
Jacqueline Tong as Mrs Todd
based on the BBC television sitcom Dad's Army