The continuing story of the Crawley family, wealthy owners of a large estate in the English countryside in the early 20th century.
The film begins in 1927. Robert Crawley, the Earl of Grantham, receives a letter from Buckingham Palace announcing that King George and Queen Mary are coming to Downton Abbey as part of a royal tour through the country. Robert first informs his daughter, Lady Mary Talbot, and son-in-law, Tom Branson. When Violet Crawley, Dowager Countess of Grantham, hears the news, she is perturbed that Queen Mary’s lady-in-waiting, Maud, Lady Bagshaw, will also be coming to Downton. Lady Bagshaw is Robert’s first cousin once removed, and the two families have fallen out over an inheritance issue.
When the royal staff arrive, including the Royal Page of the Backstairs, the Queen’s Royal Dresser, and Richard Ellis, the King’s Royal Dresser, the Downton staff are affronted by their arrogance. Lady Mary believes that Thomas Barrow, Downton Abbey’s butler, will fall short for the royal occasion. She asks Mr Carson, Downton’s retired butler, to temporarily resume his former duties, upsetting Barrow. In a moment of rare candor, Barrow strongly protests to Lord Grantham, who is impressed with Barrow’s integrity. He tells Lady Mary that Barrow will not be sacked. Downstairs, the plumber who arrives to repair the broken boiler flirts with Daisy, upsetting her fiancé, footman Andy Parker.
A man calling himself Major Chetwode arrives in Downton village. He seeks out Tom, who suspects he is a royal detective involved with security for the royal visit. Lady Mary sees Tom and Major Chetwode leaving a pub together and follows them.
Before the Royal parade through Downton Village begins, Chetwode finds the king on his horse awaiting the Royal Artillery. When he aims a pistol at the king, Tom tackles him to the ground. As Tom clamps down on Cherwode’s hand grasping the pistol, Lady Mary stomps his hand and kicks the weapon away. Now disarmed, Chetwode is arrested. Meanwhile, Tom has met and formed a mutual attraction with Lady Bagshaw’s maid, Lucy Smith.
The household is completely taken over by the royal visitors and their entourage. The king informs Edith’s husband, Bertie Pelham, the Marquess of Hexham, that he is to accompany the Prince of Wales on a three-month Tour of Africa. Edith is pleased for Bertie but also upset as she is now pregnant, and expects to give birth just as Bertie would be departing with the Prince. In the garden, Tom encounters a woman crying. Unaware that she is Princess Mary, he initiates a conversation which inspires her to remain with her intolerable husband for her children’s sake. Meanwhile, Anna discovers that Miss Lawton has been stealing from Downton Abbey. She agrees to say nothing provided Lawton returns the items and alters a ballgown for Lady Edith after the wrong garment was delivered to Downton.
Downstairs, Anna and John Bates have a plan to retake the household and restore Downton’s honour. The staff agree, though Mr. Carson has concerns. Barrow and Mr. Ellis implement the plan, tricking most of the royal staff into returning to London. The two men then go to York for a drink. While waiting for Mr Ellis to return from visiting his nearby parents, Barrow meets Chris Webster, who invites him to a club. Webster takes Barrow to a secret nightclub where the patrons are mostly men dancing together. The police storm the club, arresting everyone. Mr. Ellis discovers what happened and uses his position within the Royal Household to get Barrow released. After, the two confide about their respective homosexuality, and Ellis later gives Barrow a silver locket as a memento until they meet again.
At dinner, the king is impressed by the revised menu and praises his chef’s culinary skills. Mr. Molesley, quick to defend Downton, responds that Mrs. Patmore prepared the dinner and that the Downton staff are serving it. Everyone is stunned by Molesley speaking to the king without permission. Robert apologises for Molesley’s outburst, but the queen praises Mrs. Patmore’s cooking and tells Cora that they are accustomed to people behaving strangely around them.
Violet is upset that Maud has chosen Lucy as her heiress over Robert. However, Isobel correctly surmises that Lucy is Maud’s illegitimate daughter, which Maud confirms; she has named Lucy as her heiress out of love. Violet is understanding when told the facts. Henry Talbot, Lady Mary’s husband, arrives home in time to accompany the family to Harewood. Princess Mary tells her parents that Tom influenced her decision to remain with her husband. The king tells Tom that he has more than one thing to thank him for.
Lady Mary asks her grandmother about her recent trip to London. Violet reveals that medical tests show she has little time left to live. Violet reassures Lady Mary that Downton’s legacy will be safe in her hands. The king and queen congratulate Edith and Bertie on their forthcoming child and announce that Bertie will not go on the tour. During the festivities, Lucy enters the ballroom to bring Lady Maud a handkerchief, using it as an excuse to watch the dancing. Tom wishes they could dance together, so they leave the ballroom and dance on the terrace. Back at Downton Abbey, Mr. Carson and Mrs. Hughes discuss Downton Abbey’s future. Carson asserts it will stand for another hundred years with the Crawley family still living in it.
In the opening scene, the iconic theme plays while a letter is signed and sealed in Buckingham Palace. The letter is then couriered by footman through the royal mailroom, onto a train through the English countryside, to the post office in the village of Grantham. It is finally delivered to Downton Abbey where the Earl of Grantham (Hugh Bonneville) announces that the King and Queen will be coming to Downton for one night to visit, as part of their Yorkshire tour. There will be a luncheon, parade, dinner, and a ball.
As the news and excitement spread throughout the household, we see that Lady Mary (Michelle Dockery) is running the estate. Lady Mary’s new husband, Henry Talbot (Matthew Goode) is in Chicago for a car show and will not be able to return immediately for the royal visit. Tom Branson (Allen Leech) is still a Republican (anti-monarchist). Mr. Thomas Barrow (Robert James-Collier) is now the head butler, kitchen maid Daisy (Sophie McShera) is unenthusiastic about the royal visit as well as her engagement to footman Andy. Lady Edith (Laura Carmichael) is busy running her own household, and Mr. Carson (Jim Carter) is retired to his own cottage with Downton housekeeper Mrs. Hughes who reminds him, “you’re not in charge there anymore.”
We see a known man traveling to Grantham on the train, entering a hotel, and later going to the car shop now run by Branson and Talbot. He questions Branson’s political leanings, and we are led to believe that perhaps Branson and his Irish background and anti-monarchist political sensibilities are seen as a threat to the royal visit. Branson reassures the man that he is loyal to the Crawley family if not the King.
At the palace, we see Queen Mary (Geraldine James) speaking with her royal attendant Lady Bagshaw (Imelda Stauton) about the visit. Lady Bagshaw asks if perhaps she could not go to Downtown with the Queen and reveals the Earl of Grantham is her cousin and closest male relative, and that Lady Grantham (Maggie Smith) assumes that he will be her heir. She insinuates that she has other plans.
Preparations are in full swing, and tensions are high. When Barrow shows too much deference to the opinion of the royal servants, Lady Mary grows concerned that Barrow is not up to the task of head butler and goes to Carson and asks him to come back and resume his duties for the royal visit. Carson agrees, and Barrow is understandably offended and refuses to be part of the preparations at all. He decides to go into town with a new acquaintance from the royal service.
When the King’s chef, butler, and housekeeper arrive in advance of the King and Queen, the Downtown staff are disappointed to learn that they are being pushed aside in all aspects of service (cook, footmen, butler, maids, etc.) in favor of the royal staff.
Branson meets Lady Bagshaw’s maid, Lucy, and they form a connection over understanding that awkward place between being considered a servant versus part of the family.
Preparations continue to deteriorate. The boiler goes out. A plumber has to be called in to fix it. There seems to be a spark between Daisy and the plumber which upsets footman Andy so much that he intentionally breaks the boiler again after it is fixed the first time. Ms. Patmore hates to disappoint the grocer that he will not be supplying dinner to royalty after all. Several things of value start disappearing. Chairs have to be ordered for the parade and set up in the pouring rain. The family is informed of the tension between Lady Grantham and Lady Bagshaw over the inheritance and Lady Grantham’s plans to confront her about it during the visit.
After the initial luncheon with the King and Queen, Lady Mary’s maid Anna (Joanne Froggatt) and Mr. Bates (Brendan Coyle) present the staff with a plan to push the royal servants out of the way, so that they can serve the royalty themselves and “restore Downton’s honor.”
At the parade, Branson, with Mary’s help, foils an assassination attempt on the King by the man Branson had previously thought was keeping an eye on him but was using Branson to get information about the exact location of the King.
Before dinner, the staff works together to lure the royal footmen away to London, drug the royal chef with sleeping pills, and lock the royal butler in his room, so that they can prepare and serve the meal to the King and Queen. The meal is a great success, so much so that the King sends his compliments to his staff, and footman Moseley can’t resist piping up and announcing that it was not the royal chef who prepared the meal but Ms. Patmore and the Downton staff who served it. His faux pas is graciously overlooked by the Queen and King.
The King asks Bertie (Edith’s husband) to accompany the prince on a several-month long tour with the prince. He accepts before Edith has the chance to tell him that she is expecting their first child and doesn’t want him to be away at the birth. He tries to get out of it, but to no avail.
While in town, Barrow goes with a stranger he met in a bar to a secret gay club only to have it raided by police a few minutes later. His new friend from the royal service manages to get him out of jail. Before he departs, Barrow asks if he can write him and says he feels like he finally found a friend. He responds, “Only a friend?” and kisses Barrow.
Anna confronts the Queen’s maid about stealing all the things that have gone missing and demands that she return them and fix Lady Edith’s dress which requires significant alterations in order to fit before the ball the next night in exchange for not reporting her.
At the ball, all is resolved. Mrs. Merton (Penelope Wilton) convinces Lady Bagshaw to reveal to Lady Grantham that Lucy is her daughter and that is why she wants to leave her estate to her maid instead of the Earl of Grantham. Lady Grantham is relieved that Lucy is not a conniving swindler and starts making plans to get Tom Branson to marry Lucy and restore the estate to the family.
Lucy and Tom seem more than happy to go along with everyone’s plans. Tom asks Lucy if she would like to correspond and offers “his shoulder” in her transition from maid to heir and they are shown dancing together out on the terrace.
The King tells Bertie that he doesn’t need to go on the trip with the prince after all so he can be home for his child’s birth. We shortly discover that Cora Crawley (Elizabeth McGovern) brought about that change of heart. “He asked the king, but I asked the queen,” she tells Edith.
Princess Mary decides to stay in her loveless marriage after exchanging some wise words with Tom Branson (who didn’t realize who she was at the time) about deciding what is most important.
Daisy discovers with Andy’s passionate response to his jealous over the plumber that they do in fact, have something in common and that she will move forward in marrying him.
Finally, Lady Mary comes to a resolution with her doubts and fears about continuing the efforts to keep Downton going after having a heart-to-heart with Lady Grantham. Lady Grantham reveals she doesn’t have much longer to live, but she knows that Mary will do a fine job in carrying on her legacy. As Mary swirls around the dance floor with her husband Henry, they both reaffirm that they are “stuck” with Downton.
In the final scene, Mr. Carson and Mrs. Hughes leave through the front door of the Abbey, stating that it will be alright this once. As Mrs. Hughes wonders about the future, Mr. Carson comments that 100 years from now Downton Abbey will still be standing and the Crawleys will still be living there.