Early spring 1944. Army Captain Charles Ryder transports his company to brigade headquarters at a secret location. After an exhausting journey, they arrive at the new camp, Brideshead Castle, the site of bittersweet memories for Charles.

Charles spends time at Brideshead with Sebastian, meeting more of the Marchmains and discovering the role of religion in the family's lives. Charles accompanies Sebastian to Venice, where the Marchmain patriarch lives in voluntary exile.

Back at Oxford, Charles and Sebastian fall into a funk. The Lady enlists Charles to watch over wayward Sebastian, but Sebastian continues his drunken revelry. A crisis is narrowly averted, while Sebastian's mood continues to grow darker.

Charles tries to cover up for Sebastian's drinking, but Sebastian turns on him, accusing Charles of being a spy for his mother. Sebastian proves beyond help and leaves Oxford. Charles also leaves and chooses to study art abroad.

Charles and Sebastian both return to Brideshead for the New Year. Alcohol has been banned from the house. When Sebastian turns up drunk, Lady Marchmain blames Charles, who leaves, promising never to return.

Looking for Sebastian, Rex Mottram visits Charles in Paris and tells him that The Lady is ill and the family's fortune is dwindling. Rex announces that he plans to marry Julia. Lady Marchmain agrees, provided he converts to Catholicism.