Lord Robert Crawley and his family live a life of leisure, while a fleet of servants, including butler Carson, attend to their every need. But two events conspire to shake things up: the sinking of the Titanic, which claims Crawley's heirs, and the return of his valet, Bates. Since Crawley and Lady Cora have three daughters, his distant solicitor cousin, Matthew, becomes heir to the estate. With that, the scheming begins, since Thomas the footman views Bates as an interloper and Crawley's mother, Violet, feels the same way about Matthew's mother, Isobel.

The Great War rages across Europe, and not even the serene Yorkshire countryside is free from its effects. The men and women of Downton are doing their part both on the front lines and the home front, but the intensity of war only serves to inflame the more familiar passions: love, loss, blackmail, and betrayal.