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.
The Great War unsettles life at Downton and surprising news about Matthew rocks the family. New maid Ethel irritates O'Brien - and Bates returns with devastating news for Anna.
Carson is finding it tough maintaining standards in the house, and the suicide of a young officer prompts Isobel to suggest converting Downton Abbey into a convalescent hospital.
After Downton is turned into a convalescent home for wounded officers, Matthew brings a general to visit, but Branson has a surprise in store. And is Lavinia hiding something?
Tensions flare between Isobel and Cora. Ethel's flirtation with Major Bryant lands her in hot water. There is worrying news about Matthew. Branson declares his feelings for Sybil.
Robert receives some shocking news from the front. Daisy's relationship with William moves faster than she would like. Vera threatens to expose old secrets.
Carlisle threatens to ruin Mary if she jilts him. A major comes to stay - but is he really who he claims to be? Bates learns that Vera has succeeded in scuppering the divorce.
How long can Sybil and Branson keep their secret? Carlisle puts Anna in a difficult position, Bates is left reeling from the news about Vera, and Thomas embarks on a new scheme.
With the wedding approaching there is excitement in the air, but will Branson's plans rattle everyone? A decisive Anna forces Bates to think about their future.