1944: in the fifth year of the Second World War, a thoughtful middle-aged British Army officer, Captain Charles Ryder, finds himself posted unexpectedly to Brideshead Castle. Ryder has been there before, and time slips back to when he was an undergraduate at Oxford, twenty years before. There he meets Sebastian Flyte (a young man with a teddy bear) when Sebastian, rolling drunk, vomits through Charles's window and sends him flowers to apologize. Charles visits Brideshead for the first time.

While Sebastian recovers from a broken foot, he and Charles enjoy a long summer vacation on their own (apart from the servants) at Brideshead, drinking a lot of good wine. Then other members of the family arrive and have a lot to say about religion - the Flytes are devoutly Catholic.

Charles gets closer to Sebastian's family, with the result that he and Sebastian begin to grow apart. Then one night they get drunk, go for a drive with some girls, and Sebastian is charged with drunk driving. Julia's boyfriend Rex Mottram persuades the court that there are mitigating circumstances. Meanwhile, Aloysius seems to have been written out.

Sebastian's alcoholism, depression and misbehaviour become more and more of a problem. He is sent down (expelled) from Oxford and after an unhappy Easter at Brideshead goes abroad with a tutor, leaving Charles devastated. Charles's father agrees to his leaving the University to become a painter, so long as he also goes overseas.

Charles comes home from Paris to spend New Year 1925 at Brideshead, but Sebastian behaves disgracefully. The Marchmains are trying to stop Sebastian from drinking by keeping him penniless, and Charles incurs their anger by giving him money.

Rex Mottram is determined to marry Julia, a prospect which appals Lady Marchmain, who is fatally ill. She reluctantly agrees to a secret engagement. Then Brideshead makes his own dramatic announcement, and Julia reveals how far she has already gone in her relationship with Rex.Meanwhile, Rex is looking for Sebastian and believes Charles is hiding him.

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

Bridey is planning to demolish Marchmain House, the family's London residence, and commissions Charles to paint some last pictures of its interior before the old place comes down. Meanwhile, Charles's marriage is collapsing.

Charles and Julia meet on a transatlantic liner and begin an affair. Rex Mottram and Charles's wife conspire against them.

Bridey rebukes Julia for her affair with Charles, and Cordelia brings news of Sebastian living a dismal life in a remote monastery in North Africa.