The second series opens with Matthew in the trenches in 1916 during the Battle of the Somme. The war is raging and many men are dying. Matthew is commanding his unit well and his men respect him. Back at Downton Abbey, the family and servants are preparing for a benefit concert in order to raise funds for the war effort which involves the local hospital. Robert is having trouble managing because Bates is in London for his mother's funeral and young William is not terribly familiar with uniforms. Robert is in regimental dress because he has been made Lord Lieutenant of the Regiment and he thinks this will mean that he will soon be fighting at the front as he did when he was previously in the army.
Branson is teaching Edith to drive and she is slowly getting the hang of it but Sybil has more important things to worry about when she learns that yet another young man of her acquaintance has been killed in action. William wants very much to enlist but Mrs. Patmore is strongly against such action, even though she doesn't care to explain why. Mr. Carson is working far too hard as he has no intention of letting standards slip at Downton. Meanwhile, we learn that Matthew is engaged to a Miss Lavinia Swire, the daughter of a London solicitor, and that he is bringing her to the village to meet his mother. Cora and Violet are disappointed that Matthew didn't end up with Mary but Mary herself is in London and has not yet heard.
Sybil decides to train as an auxillary nurse to help the war effort but first receives some essential cooking tips from Mrs. Patmore and Daisy. The staff are annoyed with the new maid, Ethel, and O' Brien plays a trick on her when she tells her that Cora wishes to see her upstairs immediately to compliment her on her excellent work. Needless to say, everyone is amused except Ethel herself! Later, when Mary returns on the same train as Bates, she is devastated to hear of Matthew's engagement but puts on a brave face and only the faithful Anna is allowed to see her tears. Mary's reunion with Matthew is sad and poignant and it is clear that they still love each other but Matthew is now out of reach and his young fiancee knows she has a lot to live up to.
The benefit concert goes well until two girls present white feathers, denoting cowardice, to William and other young men not in uniform. William is upset but Branson isn't the slightest bit worried. Much better tidings are in store for Anna when John Bates finally proposes as his mother has left him a house and some money. All he has to do is get divorced and they can then make plans for their future.
Just after a very sad Mary sees Matthew off on the train, Vera Bates turns up wanting her husband back and threatening to tell the newspapers the whole sordid story of Mary and Kemal Pamuk if he doesn't agree to resign his post and leave with her. He gets very angry, but agrees as he does not want the family disgraced. Robert is furious beyond measure when he hands in his notice with no explanation but Mrs. Hughes heard the entire conversation through a grate in her office and tells Mr. Carson, who informs His Lordship that Bates resigned to protect the family honour but obviously does not tell him why.
Violet deviously makes it impossible for Molesley and William Mason to become soldiers but her trick is found out and it's obvious that William really wants to enlist but Molesley pretends to a highly unimpressed Dr. Clarkson that he cannot join up because of a lung condition. Robert is very unhappy to learn that his rank is a ceremonial one only and that he will not be joining the regiment in France after all. Thomas, who runs into Matthew in the trenches, deliberately gets himself shot in the hand by an enemy bullet so that he can leave the front as he cannot take the reality and the horror of war any longer. At the same time, Robert is extremely unimpressed that Mary seems interested in Sir Richard Carlisle, a newspaper magnate whose fodder is mainly scandalous stories. Mary wants him to come and stay at Downton so that her family can meet him, even though he is much older than she and clearly unacceptable to His Lordship.
The episode ends as we wonder what will happen with Sir Richard and Mary and will Thomas get his way and be sent home, away from the fighting.