Seven female strangers from different backgrounds meet in a train compartment heading for an ATS training camp. As they get to know each other, they experience the early stages of their new military existence: eating in the canteen, collecting their kit and uniform, marching, drilling and attending lectures.

In their beds before lights-out, they learn of each other's backgrounds, circumstances and aspirations. Maggie is a lively and optimistic Scot; Joan is an ex-dance teacher with a fussy and supercilious manner; Anne comes from an English military family; Erna is a refugee Czech, whose fiancé; father and brother have been killed by the occupying Nazi army; Gwen is a cockney café waitress; Dot has given up her job in a beauty parlour voluntarily to enlist and Betty is an only child with little experience of the outside world.

After completing basic training, they are informed of their new duties. Erna, Maggie, Anne and Joan will become drivers, Dot and Betty will be stationed at an anti-aircraft battery and Gwen is to remain at the training camp working in the mess hall.

The next day Anne, Maggie, Erna and Joan are told they will form part of a convoy of women driving lorries filled with electrical components for some vital but unspecified mission. Before leaving, they attend a dance, where Maggie meets Alexander, a kilted Scottish soldier, and Anne falls in love with David, a young pilot who is on his last days of leave. Before returning to camp he instructs his mother to befriend Anne.

The four women begin their mission and, after driving all day, stop at a roadside café prior to retiring for the night. However, a dispatch rider brings word that the secret mission has been brought forward by a day, and they will have to drive through the night.

They successfully arrive at the embarkation point, and return to their barracks by train. They share a compartment with several male soldiers, and their tiredness and the cramped conditions lead to an argument between Joan, who rashly speaks of Nazi efficiency in approving terms, and Erna, whose country has been invaded and her loved ones murdered.

Meanwhile, Dot, and Betty arrive at the battery, where they will help to aim the searchlights and operate the guns used to destroy enemy aircraft. Shortly after, Gwen also arrives to work as a telephonist. That evening, Dot reads in a newspaper that her husband has been seriously wounded in action.

Joan, now promoted to Corporal, continues to alienate her colleagues with her over-officious manner, and overhears Anne, Erna and Maggie discussing her in unflattering terms.

Next day, Anne takes Erna and Maggie to tea with David's mother. Anne reflects that this is the first war in which women are fighting side by side with men, and that it will make a tremendous difference to the status of women when peace returns. David's mother tells them of her experiences during the First World War. As an ambulance driver, she had been wounded in the shoulder by flying shrapnel, and while recuperating met her husband who had also been wounded. They were married, but he died twelve years later as a consequence of the injuries he received during the war. The following day, Anne learns that she too is to become an ambulance driver.

That night, the growing tensions between Anne and the others finally flares into outright hostility, but just as they are about to confront each other the air raid siren sounds, and they are informed that they must drive ambulances to the battery where Dot, Betty and Gwen are stationed, as it is being targeted by enemy planes. Just as they are about to leave, Anne receives a telegram informing her that David is missing, believed killed.

As they drive to their destination, Erna and Anne discuss their feelings of loss, while in the other vehicle Maggie and Joan make friends. They successfully bring down an enemy plane and avoid receiving any direct hits. The all clear sounds as dawn rises over the horizon.

Tired and with mixed emotions, the seven friends queue for breakfast together.