Set in April 1917 and with many of the male staff serving in the war, Mr. Carson finds himself under pressure to make sure that duties are carried out to his exacting standard. He ends up finding himself forced to accept female staff serving in the dining room.
With Bates still absent, Isobel's butler, Molesley, makes a play for Anna but is rejected. Robert needs to hire a new valet and ends up employing Lang, who has just returned from the war, shell-shocked. William is happy after he receives an important letter that will whisk him off to the glory of war. Mrs. Patmore finds out some devastating news about her soldier nephew, he was shot for cowardice.
Lady Edith Crawley learns to drive a tractor and spends much time helping out on the farm with Mr. Drake, who eventually kisses her to her delight, but is seen by his wife and she is shortly replaced. Thomas returns from the war and begins work alongside Lady Sybil Crawley in the local hospital. After a patient, Edward Courtenay, is brought in suffering with gas blindness, Thomas finds himself becoming attached to him but is left devastated when he commits suicide. At Isobel Crawley's suggestion - and to Violet's dismay - Downton Abbey is turned into a convalescent ward to ease the hospital's bed shortage.
Matthew Crawley learns of his temporary transfer back to England for a recruitment drive in the north whilst fighting in the trenches. Mary invites middle-aged newspaper tycoon and prospective beau, Sir Richard Carlisle, to a dinner party, also attended by Matthew and Lavinia. Suspicious of Lavinia, Violet invites her daughter Lady Rosamund, who is intrigued by the fact that Carlisle and Lavinia already seem well acquainted, and becomes suspicious of a scandal between them. Due to stress, Carson has a problem with his heart, but advises Mary that if she still loves Matthew she needs to tell him before it is too late. Mary attempts to tell him but decides not to after Lavinia expresses how much she loves him. Sir Richard proposes to Mary, and she later tells Anna that she plans to accept his proposal but seems unsure of her decision and equates it to second-best. Anna tells Mary, that she could never replace Bates, that it's either him or nothing.