In the pre-World War II era in the UK, Charles Ryder (Matthew Goode) starts to study history in Oxford, though his real wish is to become an artist. He befriends Lord Sebastian Flyte (Ben Whishaw), a homosexual and alcoholic and, like Ryder, an atheist. Both proclivities are disapproved of by the noble Marchmain family to which Sebastian belongs. Ryder becomes entranced by the family and their large and beautiful estate, Brideshead, in spite of the strict Catholicism of Lady Marchmain (Emma Thompson) and her other children, particularly the eldest son 'Bridey'.

Sebastian's father, Lord Marchmain (Michael Gambon), invites him, his sister Julia (Hayley Atwell) and Ryder to visit him and his mistress in Venice. Ryder becomes increasingly interested in Julia. When the family visit a carnival, Sebastian is shocked when he sees them kissing, wanting Ryder for himself, and ends their friendship. On their return to England Lady Marchmain tells Ryder he cannot marry her daughter since he is an atheist.

Sebastian's allowance is rescinded by his mother, who is concerned about his alcoholism. On one visit to Brideshead, Ryder gives Sebastian money with which he buys alcohol. He appears drunk at a ball to celebrate Julia's engagement to Canadian Rex Mottram. Lady Marchmain is angry with Ryder and tells him he is no longer welcome at Brideshead.

Sebastian flees to Morocco, where he lives happily in near poverty. Lady Marchmain, now terminally ill, asks Ryder to go to Morocco to bring him back, but Ryder finds he is unwell and cannot travel.

Years later, Ryder, now married and a successful painter, meets Julia on a cruise liner returning from South America. They realize they are still in love, and decide to leave their partners and live together in Italy.

Ryder returns to Brideshead, and persuades Julia's husband to end their marriage, so she can live with him. As they are leaving Brideshead however, Julia's terminally ill father, Lord Marchmain, arrives, wanting to spend his last days in his old home. During this time he regains his faith and dies a devout Catholic. Julia decides she cannot give up her old life and her faith to be with Ryder, and the two part company.

Several years later, during WW2, Ryder, now an army captain, find himself once again at Brideshead, which has been requisitioned by the army. A corporal tells him that the owner (Julia) is now serving in the reserves, and that Bridie, her brother, is dead. He visits the family chapel, where he finds a candle still burning, then leaves.

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