September 1939. Mrs. Kay Miniver (Greer Garson) and her family live a comfortable life at a house called 'Starlings' in Belham, a fictional village outside London, England. 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 (Walter Pidgeon), a successful architect. They have three children: the youngsters Toby and Judy (Christopher Severn and Clare Sandars) and an older son Vin (Richard Ney) at university. They have live-in staff: Gladys the housemaid (Brenda Forbes) and Ada the cook (Marie De Becker).
As World War II looms, Vin comes down from university and meets Carol Beldon (Teresa Wright), granddaughter of Lady Beldon (Dame May Whitty) 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.
Several months later, as the war comes closer to home with the bombing of Great Brittan, 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 May-June 1940 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 (Helmut Dantine) hiding in her garden and he holds her at gunpoint. Demanding food and a coat, the pilot maniacally asserts that the Third Reich will mercilessly overcome its enemies. She feeds him, calmly disarms him 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 realises that she would be foolish to try and 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 realises 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 England.
The villagers assemble at the badly damaged church where their vicar (Henry Wilcoxon) 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.