The film begins in 1927, about a year and a half after the TV series ended. Buckingham Palace informs Robert and Cora Crawley, the Earl and Countess of Grantham, that King George V and Queen Mary will visit Downton Abbey during a royal tour through Yorkshire. Violet Crawley, Dowager Countess of Grantham, is perturbed that Maud, Lady Bagshaw, Queen Mary’s lady-in-waiting, is included in the tour. Robert is Maud’s cousin and closest relative. The two families have long disputed who should inherit Maud’s estate.
The royal household staff arrive, including Mr Wilson, the Royal Page of the Backstairs; Mrs Webb, the Royal Housekeeper; Miss Lawton, the Queen’s Royal Dresser; Monsieur Courbet, the Royal Chef; and Richard Ellis, the King’s Royal Dresser. All but Ellis are dismissive and arrogant to the Downton servants. Upstairs, eldest Crawley daughter Lady Mary Talbot, who oversees the estate, constantly worries about keeping Downton afloat in a fast-changing world. Believing Mr Barrow cannot manage a royal visit, Mary asks Mr Carson, Downton’s retired butler, to temporarily resume his duties. Barrow, strongly protesting, steps aside.
Major Chetwode arrives in Downton Village seeking out the Granthams’ Irish son-in-law, Tom Branson. Tom assumes Chetwode is a detective assessing security for the royal visit. Prior to the royal parade starting through Downton Village, Chetwode stations himself near 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, Lady Maud’s maid, form a mutual attraction. Bertie and Edith Pelham, the Marquess and Marchioness of Hexham, also arrive at Downton. The King asks Bertie to accompany the Prince of Wales on a three-month African tour. 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 Princess Mary. He initiates a conversation which inspires the Princess to remain with her husband. Meanwhile, Anna discovers Miss Lawton is stealing small objects from Downton Abbey. She demands their return, then blackmails Lawton into altering an over-sized ballgown to fit Lady Edith after the wrong one was mistakenly delivered to Downton.
Anna and John Bates unite the staff into retaking control downstairs to defend Downton’s honour. 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 Patmore and Daisy to cook the dinner, and Mr Carson and the Downton footmen to wait at table. When the King praises the revised menu, Molesley impulsively blurts out that the Downton staff prepared the meal and are serving it. Robert apologises for Molesley’s inappropriate outburst, but the Queen says they are accustomed to people behaving strangely around them.
That same evening, Barrow and Ellis go to York. While Ellis visits his parents, Barrow waits at a local pub. A man there invites Barrows to a hidden gay nightclub. Shortly after Barrow arrives, police raid the club. Ellis, who is secretly gay, discovers what happened and uses his royal connection to secure Barrow’s release. After, the two men develop a bond, indicating they will meet again.
Back at Downton, as the inheritance conflict escalates, Isobel, Lady Merton, surmises correctly that Lucy is Maud’s secret illegitimate daughter, which is the reason she is Maud’s heir. She tells Maud she must inform Violet. The next morning, assistant cook Daisy, who doubted if footman Andy was the right man for her, realizes she does love him and begins planning their wedding. Henry Talbot, Mary’s husband, returns from abroad in time to accompany the family to Harewood House with the royal party.
During the ball at Harewood, Princess Mary informs her parents that Tom inspired her to save her marriage, prompting the King to personally thank him. Also, the Queen has persuaded the King to release Bertie from the imminent tour. Maud tells Violet about Lucy. Violet is not happy but understands, and immediately schemes to unite the two households through Lucy and Tom when told they plan to write to one another. Mary privately queries her grandmother regarding her recent trip to London. Violet confides that medical tests reveal she may die soon, but assures a distraught Mary that Downton’s legacy is safe in her hands. Tom later seeks out Lucy on the terrace to dance with her.
Mr Carson and Mrs Hughes ponder Downton’s future in the modern era. Carson asserts it will stand for another hundred years with the Crawley family still in residence.