Set in September 1912. Mathew Crawley, a young solicitor and the new heir, arrives at Downton Abbey with his mother, Isobel Crawley. They, the other Crawleys , and the servants all have difficulty adjusting to differences in class and culture whilst facing some resentment from certain family members and staff. Isobel Crawley, having worked as a nurse during the war, pressures Dr. Clarkson into performing pericardiocentesis on a hospital patient suffering from dropsy, and is made chair of the hospital, upsetting the Dowager Countess.
The heir presumptive, Matthew Crawley, and his mother, Isobel, arrive in Downton where they have been allotted a house. As a doctor’s widow, Isobel asks about the village hospital. Violet assures her it does not need help from Isobel or anyone. It is clear these two will be at daggers drawn from the start.
Cora’s maid, O’Brien, is openly contemptuous of the newcomers. She and Thomas encourage the servants to snub them as much as they dare. However, she has misjudged Cora and finds herself being reprimanded in front of the staff. Cora has further offended by taking on a local man, Alfred Molesley, as butler and valet for Matthew. Thomas is furious he has not been offered this post. Matthew finds it hard having a valet and in the process offends Molesley. It is Robert who makes Matthew aware of his new responsibilities. Even so, he cannot dissuade Matthew from taking a job with a local law firm. Violet finds this tradesman-like thinking absurd.
Meanwhile, the butler Carson, a stickler for standards and dignity, is mortified when he is confronted with his own past. Charles Grigg has been blackmailing him, revealing that before working at Downton, Carson was his stage partner, one half of the "Cheerful Charlies." Together, Robert and Bates defend Carson, and Bates gains respect from Carson as a result.
One of the housemaids, Gwen, seems to have a secret correspondent. She has received several packages and is seen hurrying into the village to post a letter. Second Footman, William, develops a crush on Daisy, but she is far too taken by Thomas to notice.
Defying Violet’s strictures, Isobel visits the hospital and sees a villager, John Drake, who is suffering from dropsy. She is determined he should benefit from the latest cure, but Dr. Clarkson would prefer to treat him in a more traditional way. At last he is persuaded, and a distraught Violet witnesses a seemingly barbaric procedure only to have to accept Isobel’s victory following Drake’s impressive recovery.
Despite her dislike of him, Violet can’t help promoting a match between Matthew and Mary. Even Isobel can see the benefits. So does Cora who is coming to like the new heir, even if she doesn’t want to. But Matthew is unconvinced and Mary is insulted by the very notion. The idea is consequently dead in the water, meaning that Violet and Cora have to resume their fight to get the entail overturned. Later, whilst talking to her sisters, Mary reveals she has a viscount’s heir in play, The Hon. Evelyn Napier.
Dr. Clarkson is nervous after Violet’s protests, but Robert supports him, proposing that Isobel will be brought on to the hospital board. The offer is made to Isobel and accepted. Isobel and Violet are consequently, if anything, even more at war than ever.