An opening montage reminds us of British life before the war. The Crowson family (father Jim, mother Elsie, daughters Phyllis and Celia, son Tom) visit the same seaside boarding house where they spend every summer and are warmly welcomed by the landlady.
The vivacious Phyllis and the shy, withdrawn Celia go to a dance on the pier, where Phyllis is chatted up by several young men. Ignored by them, Celia dances with a much older man who is as socially awkward as she is. Backstage at a variety theatre, chorus girls reminisce about pre-war life. The lights on the pier go out as night falls. The trains are crammed full of soldiers and Dunkirk refugees.
Jim Crowson complains that Phyllis hasn't written to Tom, and ridicules her ambition to join the WAAF. Afterwards, she confesses to Elsie and Celia that she has already signed up. Charters and Caldicott lay mines on the now deserted beach, while bookmakers quote odds on an aerial dogfight going on above their heads. The next day, bombed-out businesses try to maintain a semblance of normality.
The Crowsons' life is disrupted when Celia is summoned to the labour exchange. She wants to follow Phyllis into the WAAF, but instead is assigned to an aircraft components factory in the countryside alongside several very different women. She strikes up a friendship with Gwen Price, but Jennifer Knowles, smoking and overdressed, is clearly out of place and is disappointed to discover that there are no single rooms.
The next morning, factory foreman Charlie Forbes teaches them how to make individual parts, stressing the importance of each. Jennifer complains, but Charlie is unsympathetic. Celia takes to the job and earns high praise.
At a dance, Celia meets shy RAF recruit Fred Blake and they go outside to get to know each other better. He asks her to the cinema the following Thursday, and she accepts.
At the factory, though Charlie continues to be brusque with Jennifer, it's clear that he's attracted to her. During an air raid, he bodily picks her up and carries her to the shelter. Later on, when they share a train compartment, she will fake falling asleep on his shoulder, just to see his reaction.
At a concert, Gwen and Celia wait for Fred. As Beethoven's Fifth Symphony starts up, Celia rings him, and finds him oddly stand-offish. She returns to the music and fantasises about seeing him humiliated in court. He waits for her at the end of her factory shift and persuades her to have a drink in a local pub as he explains that he had to fly a secret mission over Germany, but that he thought of her constantly. She says she understands. Hesitantly, he proposes to her, and is accepted.
After a low-key wedding, Fred and Celia spend their honeymoon at the Crowson's regular summer retreat, though things have otherwise changed dramatically: deserted streets, barbed wire everywhere and landmines on the beach. The newlyweds have a room with a spectacular view of the sea and various plane skirmishes, but they only have eyes for each other.
After the honeymoon, Fred and Celia return to work. One day, Celia is woken up with the news that there is a phone call from the RAF. Terrified, she takes it, and is relieved to hear that it's Fred saying that he's found them a room. They go to visit it - it's spartan, but will do for the moment. They discuss their plans for the future, which are abruptly cut short when another phone call comes from the RAF a few days later - this time with the news that Celia had been dreading.
Despite seeing her outside working hours, Charlie is utterly professional towards Jennifer on the factory floor, which increases her cynicism. Later, she tells him that she is glad Celia and Fred got married when they did, as some people take too long. Charlie recognises the hint, but says that marriage between them would never work.
They return to the factory canteen, where the women are singing along to morale-boosting live music. Celia is wistfully thinking about planes, but eventually joins in.
