For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Charles meets Sebastian and makes several trips to Brideshead, where he is captivated by the estate and drawn into the Flyte family and all the baggage that goes with them.
Act II
Sebastian becomes an alcoholic, Julia’s marriage to Rex is on the rocks, Charles hates his wife, his paintings are easy and boring, and Lady Marchmain has died without reconciling with her son. The affair between Charles and Julia begins.
Act III
Charles and Julia eventually break up, Lord Marchmain dies, and Cordelia predicts Sebastian’s death.