Toward the end of World War II, Captain Charles Ryder has fallen out of love with the army. However, when his company is stationed at an old country manor called Brideshead Castle, he becomes overwhelmed with nostalgia. The manor belongs to the Marchmains, the family of his dear friend from Oxford, Lord Sebastian Flyte. Charles remembers his history with the family and how they shaped his life.

During his first year at Oxford, Charles develops a friendship with Sebastian that completely transforms his values. While he was initially studious and tended toward conventional, modern aesthetics, Sebastian brings a love of beauty and pleasure into his life. Throughout his first year at Oxford, he and Sebastian eschew responsibility and drink joyfully. However, Sebastian doesn’t want Charles to get to know his family. Although Anthony Blanche, a European student in their circle, tries to convince Charles that Sebastian is silly and insipid, Charles’s infatuation with Sebastian prevails.

Over the summer, Sebastian breaks his ankle and asks Charles to come to Brideshead and keep him company. Charles obliges, and they spend a very happy summer together. However, when they return to Oxford the next year, nothing feels the same. While Charles turns seriously to his study of art, Sebastian begins the year with warnings from the deans. Sebastian begins drinking to excess as a form of escaping from his responsibilities. After Sebastian spends an entire evening miserably drunk at Brideshead Castle over Easter, his mother, Lady Marchmain, attempts to convince Charles to keep an eye on Sebastian for her. Charles promises Sebastian that he won’t ally with Lady Marchmain against him. Convinced she cannot trust Charles to stop Sebastian’s drinking, Lady Marchmain threatens to make Sebastian live with the Catholic chaplain for his next term at Oxford. After Sebastian gets caught wandering the quad drunk, Lady Marchmain pulls him out of Oxford for a year. Upset and disillusioned, Charles drops out and moves to France to study art.

Charles returns to Brideshead Castle a little after Christmas that year. Sebastian’s alcoholism has worsened. While traveling through the Levant with an Oxford don, Sebastian managed to sneak away multiple times to get drunk. Accordingly, all the servants monitor how much alcohol Sebastian drinks. Charles worries that all the rules Lady Marchmain has imposed will only make Sebastian worse. Therefore, when Sebastian begs Charles to give him money for alcohol, Charles relents. Sebastian sneaks away from a hunting party to drink at a pub and ends the evening sick. Charles asks if Sebastian wants him there, and Sebastian says no. When Lady Marchmain finds out Charles gave Sebastian money, she accuses Charles of cruelty. Charles leaves Brideshead expecting to never return. However, when Charles is next in England, Sebastian’s sister Julia calls to tell him that Lady Marchmain is dying. Lady Marchmain wants to apologize to Charles and also ask him to convince Sebastian to return to England before she dies. Charles finds Sebastian in a Catholic hospital in Morocco, recovering from an alcohol-related illness. Sebastian has been living with a German man whom he likes because he requires Sebastian’s care instead of the other way around. Charles leaves Sebastian there.

Years pass, and Charles becomes a fairly successful architectural painter. He decides to do a series of abandoned buildings in Latin America and travels abroad. He reunites with his wife, Celia, in New York, and they travel by ship back to England. Charles is unhappy in his marriage, but Celia enthusiastically supports his career. On the ship, Charles runs into Julia again. Julia has been trapped in a loveless marriage to Rex Mottram, a Canadian upstart with a burgeoning career in Parliament. Rex couldn’t convert to Catholicism because of a previous divorce, meaning that he and Julia married in a Protestant fashion, which Julia always regretted. The ship passes through choppy waters for a few days, and Celia is bedridden with seasickness, leaving Charles and Julia alone for days to catch up. Their attraction to each other grows, and they begin an affair that continues after the ship docks in London. Although Celia discovers the affair around the time Charles’s new London exhibition opens, Charles believes he and Julia have found happiness.

Charles and Julia both plan divorces so that they can marry each other, causing scandal. Julia’s older brother, Bridey, refuses to bring his fiancée to see them at Brideshead because he believes she will be shocked by their living in sin, which upsets Julia. Julia worries about her sinful life but hopes by marrying Charles and having children, she can somewhat repent. Lord Marchmain returns to Brideshead because he is dying. Bridey calls a priest to perform the last rites even though Lord Marchmain has denounced all religion. Charles is incensed that the family would try to force religion on Lord Marchmain, but Julia believes it’s important to try. The priest asks Lord Marchmain for a sign of repentance, and to Charles’s shock, Lord Marchmain makes the sign of the cross. After Lord Marchmain’s passing, Julia tells Charles she can no longer marry him because of the sin of divorce. Charles understands. Years later, when Charles returns to Brideshead with the army, he prays in the newly reopened chapel. Although sad to see the house in bad shape, he decides that if the army being there caused the reopening of the chapel, then there’s hope.