In 1939, the terminally ill Lord Marchmain decides to return to Brideshead, and his children begin preparing for his death.

Bridey rebukes Julia for having affair with Charles while being married to someone else, stirring up intense emotions in her. Cordelia brings distressing news of Sebastian.

Charles and Julia meet on a transatlantic liner and begin an affair, to the dismay of Rex and Celia.

Bridey commissions Charles to paint some parts of the family's London residence before it is demolished. He meets Cordelia again, and she expresses her unhappiness with the current state of her family.

With Lady Marchmain very near to death, Charles searches French Morocco for Sebastian. He finds him, and is shocked by the state he is in.

Rex Mottram is determined to marry Julia, a prospect which a fatally ill Lady Marchmain is reluctant about. Charles learns that Sebastian stole money and disappeared.

Charles returns from Paris to spend New Year 1925 at Brideshead and finds the family keeping Sebastian penniless to prevent him from spending on alcohol.

Sebastian's alcoholism, depression, and misbehaviour become more and more of a problem. At an unhappy Easter at Brideshead, Charles tries to reason with him and is left devastated.

As Charles gets closer to Sebastian's family, he and Sebastian begin growing apart. Lady Marchmain visits the university and implores Charles to be a good influence on her rowdy son.

While Sebastian recovers from a broken foot, Charles is introduced to Sebastian's younger sister Cordelia and his older brother Brideshead "Bridey". Sebastian decides to accept an invitation to Venice by his father, Lord Marchmain, and takes Charles with him.

In the spring of 1943, disillusioned Army captain Charles Ryder stumbles upon Brideshead, once home to the Marchmain family, and recalls how he visited it for the first time twenty years ago.