The Crawleys and their servants prepare to host King George V and Queen Mary at Downton Abbey.
In 1927, about a year and a half after the TV series ended. Buckingham Palace informs Robert (Hugh Bonneville) and Cora Crawley (Elizabeth McGovern), the Earl and Countess of Grantham, that King George V (Simon Jones) and Queen Mary (Geraldine James) will visit Downton Abbey during a royal tour through Yorkshire. Violet Crawley (Maggie Smith) (Robert's mother), Dowager Countess of Grantham, is perturbed that Maud, Lady Bagshaw (Imelda Staunton), Queen Mary's lady-in-waiting, is included in the tour. Robert is Maud's cousin and closest relative. The two families dispute who should inherit Maud's estate. Violet wants to use this opportunity to ensure that Maud looks upon Robert as her own son and signs her inheritance over to him.
The royal household staff arrive, including Mr Wilson (David Haig), the Royal Page of the Backstairs; Mrs Webb (Richenda Carey), the Royal Housekeeper; Miss Lawton (Susan Lynch), the Queen's Royal Dresser; Monsieur Courbet (Philippe Spall), the Royal Chef; and Richard Ellis (Max Brown), the King's Royal Dresser. All but Ellis are dismissive and arrogant, which affronts the Downton servants.
Upstairs, Lady Mary Talbot (Michelle Dockery), the eldest Crawley daughter who oversees the estate, constantly worries about keeping Downton afloat in the modern world. Believing Mr Thomas Barrow (Robert James-Collier) (the head butler)) cannot manage a royal visit, Mary asks Mr Charles Carson (Jim Carter), Downton's retired butler, to temporarily resume his duties. Barrow strongly protests but steps aside. Andy Parker (Michael C. Fox) is the butler, who has proposed to Daisy Mason (Sophie McShera), a maid. Beryl Patmore (Lesley Nicol) is the cook. Mr Bakewell (Mark Addy) is the local grocer. Elsie Hughes (Phyllis Logan) is the resident housekeeper.
To top it off, the boiler in the household was not working and a plumber was called in to fix it. One of the maids Daisy Mason (Sophie McShera) gave more than the required amount of attention to the plumber, who fixed the boiler. Daisy was dating the house servant Andy Parker (Michael Fox), who gets jealous of the Plumber and breaks the boiler again after he leaves. As such there is no hot water, which angers Monsieur Courbet. Barrow enjoys the mess when Carson is in command and refuses to help him.
Lord Bertie Pelham and Lady Hexham Edith Pelham (Harry Hadden-Paton and Laura Carmichael) (Bertie is Edith's husband and Edith is a 2nd daughter of Robert and Cora) arrive the day before the royals are due to arrive. Later that evening the chairs for the parade seating arrive during heavy rain, and Lady Mary leads a messy group effort to position the chairs for the next day (since the villagers will start arriving from 9 am on-wards).
The weather clears and the King and Queen arrive to Downton's reception, where they are introduced to the whole Crawley family. Violet Crawley, Dowager Countess of Grantham (Maggie Smith) (& Robert's mother) exchanges cold pleasantries with the Queen's lady-in-waiting Maud Bagshaw (Imelda Staunton), who is Robert Crawley's, 7th Earl of Grantham (Hugh Bonneville) first cousin once removed (the 2 families have since fallen out).
Major Chetwode (Stephen Campbell Moore) arrives in Downton Village seeking out the Granthams' Irish son-in-law, Tom Branson (Allen Leech) (Husband to Sybil Branson, 3rd Daughter of Robert and Cora. Sybil died 7 yrs ago). Tom assumes Chetwode is a detective assessing security for the royal visit. Prior to the royal parade starting through Downton Village, Chetwode stations himself where the King is awaiting the Royal Artillery, unaware that Tom has grown suspicious. As Chetwode aims a pistol at King George, Tom tackles Chetwode, pinning him to the ground. The real royal detectives apprehend Chetwode, an Irish Republican sympathizer who erroneously believed Tom was an ally.
The royal entourage completely displaces the Downton household. Meanwhile, Tom and Lucy Smith (Tuppence Middleton), Lady Maud's maid, form a mutual attraction. To top it all off, there is a thunderstorm and then 200 folding chairs are delivered in the middle of the night, and they have to be set up that night itself for the parade, as the villagers will start arriving at 9 am in the morning.Violet begrudges Maud for her decision to bequeath her estate, Brompton, to Lucy rather than Robert.
Bertie (Harry Hadden-Paton) and Edith Pelham (Laura Carmichael), the Marquess and Marchioness of Hexham, also arrive at Downton. The King asks Bertie to accompany the Prince of Wales on a three-month Tour of Africa. The news distresses Edith, the younger Crawley daughter, who tells Bertie she is pregnant and is due just as Bertie would depart. In the garden, Tom encounters a sobbing woman, unaware she is The Princess Mary, Viscountess Lascelles (Kate Phillips) (the daughter of King and the Queen). She laments her failing marriage (to Henry (Andrew Havill), Viscount Lascelles), but Tom manages not only to lift her spirits but also impart wisdom that motivates her to remain with her husband.Meanwhile, Anna Bates (Joanne Froggatt) (another maid of the house) discovers Miss Lawton is stealing small objects from Downton Abbey. She demands their return, then blackmails Lawton into altering Lady Edith's ballgown by the next day, as the wrong garment was delivered to Downton.
Anna and John Bates (Brendan Coyle) unite the staff into retaking control downstairs and defending Downton's honor. Barrow and Mr Ellis trick some royal household staff into returning to London. Anna slips a strong sleeping aid into Courbet's tea, and Mr Wilson is "accidentally" locked inside his room, allowing Mrs Beryl Patmore (Lesley Nicol) and Daisy Mason (Sophie McShera) to cook, and Mr Carson and the Downton footmen to wait at table. When the King praises the revised menu, Joseph Molesley (Kevin Doyle) (Downton staff) impulsively blurts out that the Downton staff prepared the meal and are serving it. Robert apologizes for Molesley's outburst, but the Queen says they are accustomed to people behaving strangely around them.
That evening, Barrow and Ellis visit York. Ellis visits his parents while Barrow waits for him at a pub. A man at the pub invites Barrow to an underground gay nightclub, he agrees and shortly after his arrival the police arrive to cite and arrest the attendants. Ellis, also a closeted gay man, arrives at the club and uses his position in the royal household to negotiate Barrow's freedom.Later, Lady Merton, Isobel Grey (Penelope Wilton) (And Violet's friend) deduces that Lucy is Maud's illegitimate daughter which is why she stands to inherit Maud's estate. Isobel challenges Maud to explain the situation to Violet.
The next morning, Henry Talbot (Matthew Goode) (Mary Talbot's husband) returns home after traveling abroad and joins the Crawleys at Harewood House along with the royal family. During the ball that evening at Harewood, Princess Mary informs her parents that it was Tom who saved her marriage by convincing her to remain with her husband, and she encourages the King to speak with Tom. He approaches Tom and expresses gratitude not just for what he did for the Princess but indicates that he's also aware of Tom's role in thwarting the assassination attempt earlier at the parade.
The King also kindly releases Bertie from his obligation to join the Prince of Wales on a three-month tour of Africa, which the King had requested earlier in the visit. Bertie had accepted, not knowing that Edith was pregnant, and wished to change his mind after learning the good news. Thanks to Cora Crawley, Countess of Grantham (Elizabeth McGovern) and the Queen's intervention, the King had a change of heart and rescinded his request.Meanwhile, Maud privately confesses to Violet that Lucy is indeed her daughter and that's why she was named heir of Brampton. Violet kindly understands and the two reconcile, then she begins to plot a union of the two families through Tom and Lucy, as she had taken notice of the two's budding romance earlier.
Finally, Lady Mary quietly asks Violet about her visit to London. Violet informs her that she had gone for a doctor's visit, during which tests revealed that she is in fact, dying. Mary is distraught, but Violet assures her that Downton and its legacy is safe in Mary's hands. As this conversation carries on, Tom finds Lucy on the terrace and the two dance in the dark listening to the music played inside at the ball.
