It is May 1940, and the German forces have just invaded Holland, Belgium, and Luxembourg. Winston Churchill, 65 years old, is Britain’s topmost naval official, the first lord of the Admiralty. Scotland Yard detective Walter Henry Thompson acts as Churchill’s body guard. The British House of Commons is rocked by a rebellion against the current prime minister, Neville Chamberlain, for his appeasement policies toward the Nazi regime and a failed military intervention against the Germans in Norway. Chamberlain is forced to resign and recommends Churchill as his successor, to which the king gives his assent. Churchill’s elation at the prospect of being prime minister is balanced against the awareness that dark days lie ahead and increasing personal challenges involving his family: his loyal wife, Clementine, his wayward son, Randolph, and other sons and daughters.

In the midst of sober war preparations, Churchill’s teenaged daughter Mary attends a jovial party given by her friend Judith Venetia Montagu. Although the young members of Britain’s aristocracy enjoy nightlife and partying, the impending war brings out a more responsible side to them. The men join various branches of the military, while the women help the war effort by resettling evacuees and providing food and clothing.