The story follows two British soldiers through forty years of warfare. The framing story follows a Home Guard exercise in which Spud Wilson captures Clive Wynne Candy in a Turkish bath, six hours before the exercise is supposed to begin. Candy insists that Wilson is not playing by the rules, but Wilson argues that rules should not be followed in modern warfare. As Candy dresses down Wilson, his tale is told is flashback.
In 1902, during the Boer war, Candy travels to Berlin where he confronts a German propagandist, Theo Kretschmar-Schuldorff and challenges him to a duel. As they recuperate from the wounds incurred in the duel, the two become friends. Candy's traveling companion, Edith Hunter then tells him that she and Theo plan to marry. It is only then that Candy realizes he's in love with Edith. During World War I, Candy adheres to the gentlemanly fighting that he carried on during his Boer War campaigns and he marries Barbara, a nurse who bears a strong resemblance to Edith. Candy then discovers Theo is a prisoner-of-war in England. Gradually the two friends renew their acquaintance. At the start of World War II, Theo is back in England as a German refugee, escaping from the Nazis. He meets Candy, who is now widowed and pensioned out of the War Office because of his old-fashioned ideas of warfare. Theo convinces Candy to join the Home Guard, but after Candy ruins the exercise, he sadly feels that he has nothing more to offer his country. But Theo tells him that new ideas of warfare are required for this new war and Candy, as a pristine example of British soldiery, still has much to offer in the life or death fight against the Nazis.