In the spring of 1944, Capt. Charles Ryder finds that he and his men are relocated to the grounds of Brideshead Castle. Charles knows the place well and he recalls a time 20 years before when he met Lord Sebastian Flyte, the second son of Lord Marchmain and for whom Brideshead was home. It was in his third term at Oxford that he and Sebastian became great friends and for Charles, a time when he matured and gained a greater appreciation for life. He soon became part of Sebastian's group of friends including such characters as the flamboyant Anthony Blanche and Boy Mulcaster. During the long holiday, Charles finds himself living at home with his somewhat eccentric father, with no money and little to do. He is saved however when Sebastian asks him to come to Brideshead for a visit. There, he meets Sebastian's sister Julia for the first time.

The aristocratic Sebastian invites his middle-class university friend Charles to spend a glorious summer at the amazing family home, Brideshead.

Charles and Sebastian return to Oxford but feel old and out of place. Sebastian feels that his mother is constantly watching him through her friends. In particular, both young men have to put up with Mr. Samgrass who is obviously reporting on all of their activities. Sebastian is drinking far too much and neither are enjoying themselves as they did during previous terms. They also get into a bit of trouble when Boy Mulcaster suggests they go to a private nightclub for a bit of fun. Later, they are arrested for drunkenness but for Sebastian, the charge is more serious as he was behind the wheel of a car. They are bailed out by Julia's friend, Rex Mottram and Sebastian manages to avoid jail.

Sebastian's decline continues and there is little anyone seems able to do about it. He is terribly unhappy about his family situation and seems bent on destroying any relationships he may still have left. Soon he is sent down and Charles subsequently learns that Sebastian and Mr. Samgrass will be departing for a tour of Middle Eastern monasteries. Change comes for Charles himself when he decides not to return to Oxford but rather to pursue perfecting his talents as an artist. He attends an art school in Paris and hears little of Sebastian during these months. Throughout it all however, he remains loyal to his good friend despite Lady Marchmain's attempts to co-opt him.

Charles is invited to Brideshead for Christmas and learns that Mr. Samgrass and Sebastian will be there as well. Sebastian is as morose as he was before he left, if not more so. The atmosphere as Brideshead is gloomy. All of the liquor has been locked away and there is a tension among everyone there due to Sebastian's condition. The more his mother tries to exert control over him, the more Sebastian travels in his downward spiral. Charles remains loyal however and gives Sebastian money knowing that he plans on going to a pub. When Lady Marchmain asks him if he was the source of the money he is honest with her and her harsh words convince Charles that he will never again return Brideshead.

Charles returns to Paris to continue his art studies and is surprised with a visit from Rex Mottram. It would seem that he and Sebastian had stopped in Paris enroute to a sanatorium in Zurich but Sebastian has stolen his cash and disappeared, much as he did with Mr. Samgrass during the tour of the Middle East. Charles recounts the tale, learned many years later, of Rex and Julia's relationship. Much to Lady Marchmain's objection, Rex and Julia are engaged and Rex agrees to take instruction in the Roman Catholic faith. Rex very much wants a big society wedding but only a short time before the event, Bridey learns that that Rex was once married and is now divorced. Not realizing the impediment this causes Roman Catholics, Rex has some difficulty understanding why they can't just go ahead. Julia stands her ground however and insists they will be married one way or another.

Charles returns to London in the Spring of 1926 during the General Strike and joins the Defense Corps which ensures the delivery of essential supplies. He meets up with Boy Mulcaster and Anthony Blanche. Charles learns that Sebastian is living in French Morocco with a German boy he'd met. Lady Marchmain is dying and on her mother's behalf, Julia asks Charles if he would go and retrieve Sebastian. Charles travels to North Africa and finds Sebastian, who is in a very bad way. He's been hospitalized with the grippe and his alcoholism is taking its toll.

Charles returns to London, leaving Sebastian in Morocco. Bridey gives Charles a commission to paint a number of small oils of Marchmain House, the family's London townhouse which is about to be torn down. Having successfully established himself as an artist Charles sets off for Central America sketching and painting for two years. Returning to New York he is reunited with his wife Celia. On the ocean liner returning to England, he meets Julia.

With Celia seasick, Julia and Charles spend all of their free time together and soon begin an affair. He also learns from her that Sebastian has vanished completely. In London, Charles' new exhibition is a success, but his reluctance to go home finally brings an end to his marriage. He meets Anthony Blanche who takes him to a gay club for a drink and promptly proceeds to disparage his work. Charles accompanies Celia to Brideshead where Julia and Rex now live and where Rex continues to entertain the political set.

Two years have passed and Charles and Julia are still in love, spending as much time together as possible. They are astounded to learn from Bridey that he is engaged to be married. Bridey's admonition of Julia's relationship and his refusal to bring his fiancée to Brideshead has a profound effect on her, however. Charles and Julia each decide to divorce their spouses and get married. Cordelia returns to Brideshead after several years volunteering in Spain during the Civil War. Charles learns from her of Sebastian since he last saw him and that he is now living in Tunis in a monastery.

In the winter of 1939, Lord Marchmain decides to return to Brideshead from Venice in view of the deteriorating international situation. It soon becomes apparent that he is in declining health and has in fact come home to die. He has taken a dislike to Bridey's new wife and suggests he may leave Brideshead Castle to Julia and Charles. Religion - specifically whether Lord Marchmain will accept the last rites - becomes an issue. Charles simply cannot understand or accept the family's religious beliefs but in his dying moments, Lord Marchmain does provide a sign indicating his wishes. The entire episode forever changes Charles and Julia's relationship. In the spring of 1944, Charles, now an army Captain, living on the grounds of Brideshead Castle, walks the corridors of a place he knows so well and remembers the life that is past him.