Sarajevo, June 1914. As Archduke Franz Ferdinand is driven through the city in an open top car, an assassin runs out of the crowd and shoots him.

In England, Sir Francis and Lady Villiers sit down to breakfast with their daughter Ann and read the morning papers. They attach little importance to the shooting in Sarajevo, especially Ann, who is more interested in sporting results. More engaging is a letter from the Villiers' son, Robin, who is at school in Germany. After looking at the enclosed photo of Robin with his friend Fritz, Ann leaves to play tennis and is driven away by Marshall, the family chauffeur.

In Germany, Robin fences with a fellow student under the approving eye of his teachers. One tells him that war is approaching and warns him to return to England as soon as possible. Robin protests that he is English and that any war is unlikely to affect him.

Back in England, the impending war is finally taken seriously and the anxious Villiers family chart the progress of international events. Robin arrives safely back in London and goes straight to the recruitment office, where many men cheat during their health checks and lie about their ages in order to enlist. Robin does the same, telling the recruitment officer that he is over nineteen.

Marshall tells the Villiers he has joined the army and is congratulated by Ann and her father. The family are less pleased when Robin returns home and admits that he has enlisted too. The new army recruits undergo training, gradually moving from disorganisation to efficiency and soon Robin is due to leave for France, much to the dismay of his mother.

In the Café Normand, near to the Front, French and British soldiers socialise, overcoming the language barrier with gesture, dance and songs. Robin greets Marshall who is now a sergeant. A group of refugees enter the café, and Robin buys a meal for a sad young woman who is among them.

In Britain, people adjust to life on the Home Front with its rations and air raids. The Villiers open their house to recovering soldiers and Marshall finds himself back in their home after being wounded in battle. He receives a letter from Robin, announcing his intention to marry the refugee he met in the Café Normand and asking Marshall to keep the news from Robin's parents. As Robin returns to the Front after a period of leave, his new wife tells him they are to have a baby.

During his period of convalescence, Marshall gradually becomes part of the Villiers family. A family friend visits one evening and tells them that Marshall is to receive an award for bravery. Mrs Villiers and Ann kiss and congratulate Marshall, much to his embarrassment. He tells Sir Francis of his intention to apply for a permanent commission once the war is over. As an air raid begins a telegram is delivered, announcing Robin's death in action. The family deal with their grief in different ways: Mrs Villier's goes to Robin's room and remembers her son as a carefree young man, while Marshall comforts the sobbing Ann.

As the war draws towards its end, Marshall returns to France to bring Robin's wife and new baby to England. The Armistice is reported in London, but the grieving Sir and Lady Villiers are oblivious to the celebrations on the streets of the city. Marshall brings Robin's child to the house and tells them that Robin got married in France. In the streets, Robin's wife gets lost in the confusion of the jubilant street party. A policeman escorts her to the Villiers house, where the family welcome her and thank her for making Robin happy.

One year later, Britain solemnly remembers Armistice Day and the Villiers honour Robin. Ann and Marshall are now in love and Marshall visits Sir Francis to ask his permission to marry his daughter. Looking away from the commemoration outside, Sir Francis acknowledges the couple's presence but does not answer their question.