The film begins in 1927 by following the journey of a letter from Buckingham Palace to Downton Abbey. As the mail is being sorted and loaded onto the Mail Train in London, a stranger boards a carriage. The letter arrives at Downton Abbey, received by Andy, the footman and handed over to Mr Thomas Barrow, the Butler. Robert Crawley, The Earl of Grantham, is given the mail as soon as he comes downstairs for breakfast with his daughter, Lady Mary Talbot, and son-in-law, Tom Branson. Robert reads the letter and announces that King George V and Queen Mary are coming to Downton as part of a royal tour through the country. Queen Mary’s Lady-in-Waiting, Lady Maud Bagshaw, is the cousin of Lord Grantham, however the two families have fallen out due to an inheritance issue. The Dowager Countess of Grantham, Violet Crawley, is told by her son Robert that Maud is coming to Downton. She is shocked and perturbed by the news. Violet believes Robert is being cheated out of a rightful inheritance. The Royal Staff arrive; including the Royal Page of the Backstairs, the Queen’s Royal Dresser and the King’s Royal Dresser, Mr Richard Ellis. The Downton staff is caught off guard by the arrogance of the Royal staff.
The stranger who boarded the train has arrived in Downton Village and books a room overlooking the village greens and seeks out Tom Branson, introducing himself as Major Chetwode. Branson is convinced Major Chetwode is one of the royal detectives just checking up on him for the visit but Mary sees Tom coming out of a pub with Major Chetwode and becomes suspicious. Before the Royal parade through Downton Village, Tom is followed by Mary, in pursuit of Chetwode who has found the King on his horse awaiting the Royal Artillery. Chetwode removes a pistol from his coat and to aims it at the King when Tom tackles him to the ground telling Mary to grab the gun.
Mary has become convinced Barrow is not stepping up to the mark and asks Mr Carson, the now retired former butler of Downton Abbey, to return temporarily. Barrow, upset with being replaced, makes his feelings clear in a moment of rare candor between himself and Lord Grantham. Before leaving he informs them that the boiler is broken. Mary asks if Barrow will be sacked however, Robert is impressed with Barrow’s integrity. The plumber arrives to fix the boiler and flirts with Daisy, setting off jealousy in her fiancé Andy, who purposely breaks the boiler again in anger.
The household has been completely taken over by the Royal Staff but Anna and Bates have a plan. The King asks Bertie to accompany the Prince of Wales on a 3-month Tour of Africa; Edith is excited for Bertie but upset as she is now pregnant. Princess Mary is outside on a bench in the garden crying; Tom, unaware of her identity, approaches her, sparking a conversation which inspires the Princess to stay with her intolerable husband for the sake of the children. Edith’s ball gown arrives however, in a delivery mix up, she receives a similar dress for someone twice her size. Anna, realizing that Miss Lorton is stealing from Downton Abbey, agrees to say nothing provided that Lorton return the items and fit Edith’s dress by morning. Downstairs, the Downton staff has gathered in the cellar excited by a plan to save Downton’s honour, but Mr Carson is concerned.
Mr Barrow and Mr Ellis trick most of the royal staff into traveling to London as part of Anna and Bates’ plan to take back the household. The two then continue into York for a drink, after which, Barrow waits for Ellis to return from visiting his nearby parents. While waiting, Thomas Barrow meets Chris Webster, who invites him to a club. Barrow accepts and leaves a message for Ellis. Webster takes Barrow to a hidden nightclub called the “Turton”. The crowd inside is mostly men dancing together. Barrow and Webster begin to dance when the police storm the nightclub, throwing everyone into police vans. Mr Ellis witnesses the arrests and as the King’s Royal Valet gets Barrow released where the two confide in each other about their homosexuality. Later, Ellis gives Barrow a silver locket as a reminder until they meet again and kisses him.
At dinner the King comments on the difference from the planned menu and is impressed, praising Monsieur Courbet. Mr Molesley, quick to defend Downton, stops as he is carrying away a tray, to say that the dinner was actually prepared by Mrs Patmore and that the Downton staff were serving it. There is a stunned moment of absolute silence as Molesley realizes he just spoke to the King without being asked. Extremely surprised and embarrassed, Robert apologizes. The Queen, understanding his embarrassment, praises the work of Mrs Patmore and tells Cora that they are used to people behaving strangely around them. Isobel has figured out that Lucy is Lady Bagshaw’s daughter. Maud explains to Isobel that she never married Lucy’s father before his death but has made Lucy her sole heir. Isobel insists that she tell Violet the truth. Maud reveals the truth to Violet, who is understanding. Princess Mary tells her parents that she will stay with her husband and that Branson was an influence over her decision. She asks her father to say something nice to Branson. The King responds by approaching Tom and telling him that he knows he “has more than one thing to thank him for”.
Mary asks her grandmother about her journey to London. Violet reluctantly tells Mary that she has had medical tests and doesn’t have much longer to live. Mary, while extremely upset, is calmed by Violet who puts her at ease, telling Mary how proud she is of her. Henry Talbot arrives in time to accompany the family to Harewood where Mary asks if they should keep Downton Abbey. Henry replies “we’re stuck with it, aren’t we?” Edith and Bertie are congratulated on their forthcoming child by the royal couple and told by the King that Bertie will not be going on tour. They ask Cora and Robert about the Kings change of mind. Cora tells them, that while Bertie asked the King, she asked the Queen. Lucy enters the ballroom with a handkerchief for Maud as an excuse to watch the dancing. Tom admits that he wishes they could dance together. Lucy leaves the ballroom followed by Tom and the two have their dance on the terrace. Back at Downton Abbey, Mr Carson and his wife Mrs Hughes discuss the future of Downton. Carson promises that it will be standing for another hundred years, with the Crawley family still living in it.