Kay Miniver and her family live a comfortable life at a house called 'Starlings' in Belham, a fictional village outside London. The house has a large garden, with a private landing stage on the River Thames at which is moored a motorboat belonging to her devoted husband Clem, a successful architect. They have three children: the youngsters Toby and Judy and an older son Vin at university. They have live-in staff: Gladys the housemaid and Ada the cook.
As World War II looms, Vin comes down from university and meets Carol Beldon, granddaughter of Lady Beldon from nearby Beldon Hall. Despite initial disagreements—mainly contrasting Vin's idealistic attitude to class differences with Carol's practical altruism—they fall in love. Vin proposes to Carol in front of his family at home, after his younger brother prods him to give a less romantic but more honest proposal. As the war comes closer to home, Vin feels he must "do his bit" and enlists in the Royal Air Force, qualifying as a fighter pilot. He is posted to a base near to his parents' home and is able to signal his safe return from operations to his parents by cutting his engines briefly as he flies over the house. Together with other boat owners, Clem volunteers to take his motorboat to assist in the Dunkirk evacuation.
Early one morning, Kay unable to sleep as Clem is still away, wanders down to the landing stage. She is startled to discover a wounded German pilot hiding in her garden and he holds her at gunpoint. Demanding food and a coat, the pilot aggressively asserts that the Third Reich will mercilessly overcome its enemies. She feeds him, calmly disarms him when he collapses, and then calls the police. Soon after, Clem returns home, exhausted, from Dunkirk.
Lady Beldon visits Kay to try and convince her to talk Vin out of marrying Carol on account of her granddaughter's comparative youth. Lady Beldon is unsuccessful and admits defeat when Kay reminds her that she, too, was young when she married her late husband. Lady Beldon concedes defeat and realizes that she would be foolish to try to stop the marriage. Vin and Carol are married; Carol has now also become Mrs Miniver, and they return from their honeymoon in Scotland. A key theme is that she knows he is likely to be killed in action, but the short love will fill her life. Later, Kay and her family take refuge in their Anderson shelter in the garden during an air raid, and attempt to keep their minds off the frightening bombing by reading Alice's Adventures in Wonderland, which Clem refers to as a "lovely story" as they barely survive a bomb destroys parts of the house. They take the damage with nonchalance.
At the annual village flower show, Lady Beldon silently disregards the judges' decision that her rose is the winner, instead announcing the entry of the local stationmaster, Mr. Ballard (Henry Travers), named the "Mrs. Miniver" rose, as the winner, with her own rose taking second prize. As air raid sirens sound and the villagers take refuge in the cellars of Beldon Hall, Kay and Carol drive Vin to join his squadron. On their journey home they witness fighter planes in a 'dogfight'. For safety, Kay stops the car and they see the German plane crash. Kay realizes Carol has been wounded by shots from the plane and takes her back to 'Starlings'. She dies a few minutes after they reach home. Kay is devastated. When Vin returns from battle, he already knows the terrible news. Unexpectedly he is the survivor, and she the one who gives her life for Britain.
The villagers assemble at the badly damaged church where their vicar affirms their determination in a powerful sermon:
We in this quiet corner of England have suffered the loss of friends very dear to us, some close to this church. George West, choirboy. James Ballard, stationmaster and bellringer, and the proud winner only an hour before his death of the Beldon Cup for his beautiful Miniver Rose. And our hearts go out in sympathy to the two families who share the cruel loss of a young girl who was married at this altar only two weeks ago. The homes of many of us have been destroyed, and the lives of young and old have been taken. There's scarcely a household that hasn't been struck to the heart. And why? Surely you must have asked yourselves this question? Why in all conscience should these be the ones to suffer? Children, old people, a young girl at the height of her loveliness? Why these? Are these our soldiers? Are these our fighters? Why should they be sacrificed?
I shall tell you why. Because this is not only a war of soldiers in uniform. It is the war of the people, of all the people. And it must be fought not only on the battlefield but in the cities and in the villages, in the factories and on the farms, in the home and in the heart of every man, woman and child who loves freedom. Well, we have buried our dead, but we shall not forget them. Instead they will inspire us with an unbreakable determination to free ourselves, and those who come after us, from the tyranny and terror that threaten to strike us down. This is the People's War. It is our war. We are the fighters. Fight it then. Fight it with all that is in us. And may God defend the right.
A solitary Lady Beldon stands in her family's church pew. Vin moves to stand alongside her, united in shared grief, as the members of congregation rise and stoically sing "Onward, Christian Soldiers", while through a gaping hole in the bombed church roof can be seen flight after flight of RAF fighters in the V-for-Victory formation heading out to face the enemy.
The film went into pre-production in the autumn of 1940, when the United States was still a neutral country. The script was written over many months, and during that time the USA moved closer to war. As a result, scenes were re-written to reflect the increasingly pro-British and anti-German outlook of Americans. The scene in which Mrs. Miniver confronts a downed German flyer in her garden, for example, was made more and more confrontational with each new version of the script. It was initially filmed before the December 1941 attack on Pearl Harbor brought the USA into the war, but following the attack, the scene was filmed again to reflect the tough, new spirit of a nation at war. The key difference was that in the new version of the scene, filmed in February 1942, Mrs Miniver was allowed to slap the flyer across the face. The film was released 4 months later.
Wilcoxon and director William Wyler "wrote and re-wrote" the key sermon the night before the sequence was to be shot. The speech "made such an impact that it was used in essence by President Roosevelt as a morale builder and part of it was the basis for leaflets printed in various languages and dropped over enemy and occupied territory." Roosevelt ordered it rushed to the theaters for propaganda purposes.
There is a parallel story concerning the Dunkirk evacuation. Sub-Lieut. Robert Owen Wilcoxon of the Royal Naval Volunteer Reserve, only brother of Henry Wilcoxon, assisted in the Dunkirk evacuation on 29 May 1940; but, having helped to get hundreds of Allied troops off the beach to safety in his assault landing craft, he was fatally injured when, after returning to the sloop HMSBideford to arrange a tow back to Dover, the ship had its stern blown off by a bomb dropped from a dive-bombing German aircraft. This must have been on Wilcoxon's mind during the making of the film.
This remarkably touching wartime melodrama pictorialises the classic British stiff upper lip and the courage of a middle class English family (headed by Greer Garson and Walter Pidgeon) amid the chaos of air raids and family loss. The film's iconic tribute to the sacrifices on the home front, as movingly directed by William Wyler, did much to rally America’s support for its British allies.
The film exceeded all expectations, grossing $5,358,000 in the US and Canada (the highest for any MGM film at the time) and $3,520,000 abroad. In the United Kingdom, it was named the top box office attraction of 1942. The initial theatrical release made MGM a profit of $4,831,000, their most profitable film of the year.
Of the 592 film critics polled by American magazine Film Daily, 555 named it the best film of 1942.