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.
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.
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.