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.

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.

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.

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.

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.

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.

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.

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

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.

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