On the eve of World War II, the Cazalet clan assembles at the family estate in Sussex, England. The drama centers on three brothers and a sister (Hugh, Edward, Rupert, Rachel), their spouses (Villy, Sybil, Zoe) and their children, from infants to adolescents coming of age in an ominously changing world from 1937 to 1947.
As war is declared with Germany, Rupert, who has given up his artist's career to join the family firm, now signs up with the navy. Edward joins the RAF as a ground officer in the Battle of Britain. Hugh elects to stay with the firm and serve as an air raid warden during the Blitz. Meanwhile, Zoe is pregnant again after the death of her baby, and Edward's daughter Louise learns of his affair with Mrs. Macintosh.
Rupert is reported missing during the Dunkirk evacuation. The family is resigned to his death, but his daughter Clary believes he is alive and fighting for the French resistance. Louise, now a drama student, catches the eye of handsome Michael Hadleigh, a famous portrait painter. Rachel falls out with her lesbian lover. During the Blitz, the family sawmill is obliterated.
With a new baby and a missing husband, Zoe volunteers at a hospital and befriends a horribly disfigured airman. Christopher, the 18-year-old cousin of the Cazalet children, announces his pacifism and refuses to enlist, incurring the wrath of his father, a wounded veteran of World War I. Refusing to give up hope for her father, Clary writes General DeGaulle for information on his whereabouts.
While working on airfield construction, Christopher has a mental breakdown and is hospitalized, not knowing his name. Sibyl and Hugh struggle with her terminal cancer, which each knows about but won't discuss with the other. Edward's mistress announces she is pregnant. Clary learns her father has been seen in France and wedding bells ring for Louise and Michael. America enters the war, ending the darkest days for Britain.