In 1943, a group of Home Guard soldiers on exercises, led by Lieut. 'Spud' Wilson, receive a telegram telling them that 'war starts at midnight'. Deciding to steal a march on the 'enemy', they begin early and advance on their objective, a Turkish Baths in Piccadilly, where they take captive the elderly General Wynne-Candy, who is incensed at Wilson's insubordination. They argue, then tussle, and fall into the baths.
Clive Candy emerges from the same baths in 1902 a young man, just returned from South Africa, where he has won a Victoria Cross in the Boer War. His friend 'Hoppy' Hopwell shows Candy a letter from the sister of the governess of a friend's niece, Edith Hunter. It tells that Kaunitz, with whom Candy was imprisoned in South Africa, is spreading black propaganda about English tactics in South Africa. Candy visits Betteridge in the War Office and asks to be allowed to go to Berlin to counter Kaunitz's propaganda. Betteridge advises him to stay out of politics.
Candy goes to Berlin anyway. In the Kaiserhof Hotel he meets Edith and they go to a café to track down Kaunitz. Candy has already been advised by his friend Fitzroy at the British embassy not to cause trouble because of a forthcoming visit by the Prince of Wales. Candy disconcerts Kaunitz by asking the orchestra to play a tune familiar to both of them from their imprisonment. Kaunitz confronts Candy, and in the ensuing fracas Candy insults the German Army for its associations with Kaunitz.
A duel is negotiated between Candy and an officer picked by lot, Theo Kretschmar-Schuldorff, to defend the honour of the German army. They fight with sabres and Candy wins, but injures himself afterwards falling through a window. Edith nurses Candy in the same nursing home as Theo, who is recovering from his duelling injuries. The three become friends, but when it is time for Candy to return home, Edith is upset. Theo has declared his love for Edith, and she reciprocates. Candy is delighted for them, and when he returns to England, she stays to marry Theo. Back in London, Candy is given a dressing down by Betteridge, who invites him to dinner. Candy declines.
In 1918 Candy is a Brigadier-General, trying to make his way home across France. He inspects captured German soldiers from Theo's regiment but they will tell him nothing of Theo. Unable to get transport, he finds a meal at a convent, where he notices a nurse from Yorkshire who bears a striking resemblance to Edith. Later, he receives a telegram telling him that the war is over.
In peacetime, Candy finds the nurse, Barbara Wynne, and marries her. They move into his aunt's old house in London. They learn that Theo is interned in a camp in Derbyshire and attempt to visit him but he snubs them. Months later, as Candy is having a dinner party, he receives an apologetic call from Theo who is about to return to Germany. Candy brings him to the dinner party where the guests commiserate with Theo and insist that England wants to see Germany back on her feet. On the train back with his fellow officers, Theo is contemptuous of the English attitude.
In the years between the wars, Barbara and Edith both die. Theo leaves Germany, and comes to England as a refugee: Nazism holds nothing for him. 1939 sees him registering as an enemy alien. He is about to be refused residence when Candy, now General Wynne-Candy arrives to stand surety for him. They dine, and Candy admits to having been in love with Edith since he left Berlin. Theo is given a lift back to his home by Candy's driver, Angela Cannon, who also looks just like Edith.
In 1943, about to talk about Dunkirk on a radio programme, Candy is informed that his planned address would be impolitic and he will not be allowed to make it. Back at his house, a letter from the War Office informs him that he is to retire. Theo explains to him that the planned talk in which he declares that he would rather that England see defeat than fight underhand like the Germans, is no longer appropriate in the battle against Nazism. This is no gentleman's war. They persuade him to join the newly-formed Home Guard, where he becomes a teacher and plans an exercise in which 'war starts at midnight'.
After the scene at the baths, Theo visits Candy and confesses that he finds Wilson's trick amusing. Candy decides not to make Wilson's life difficult over the incident. By the ruins of his house, which has been bombed and the site turned into an emergency water tank, he muses on the man that he has been and the man that Wilson could become. He will invite him to dinner.
