Warning: wikipedia contains spoilers
The plot concerns Charles Ryder, who, as a student at Oxford, becomes acquainted with Sebastian Flyte, younger son of an aristocratic family. Sebastian takes him to his home, Brideshead, where he meets the rest of the family, including Sebastian's sister, Julia.
Sebastian's family are Catholic, but his father, Lord Marchmain, has left his mother and gone to live in Italy with a mistress. Religion is a mainstay of the family, and becomes the ruling factor in Ryder's relationship with them. Having lost touch with Sebastian, who descends into alcoholism, the divorced Ryder eventually forms a relationship with Julia, who has separated from her husband. They plan to marry until Julia, motivated by her father's dying return to the Catholic faith, decides that she can no longer live in sin.
During the Second World War, Ryder, now an army officer, is billeted at Brideshead, where he sees how the house, like the family who own it, has been affected by time and by the change in society.