The story of Downton Abbey isn't about a family. It's about a house.
The titular character isn't flesh and blood; it's stone and brick, a building that has stood for at least a century or two when fans first lay eyes on it. The Downton wiki does not go into the history of the estate. But if one were to look to the real Highclere Castle as a model, there has probably been some sort of house standing on the estate since the 9th or 10th century. In the world of the show, the current Earl, Robert Crawley, is the 7th of his title, and at one point it is said that the house came into the family's possession under the 3rd Earl. The point is, it's been there a long time, and as Carson observes at one point in the film, it will assuredly stand at least 100 years more.
The show sometimes seemed to forget this. In the early seasons, it was less about the Crawley family as a unit and more about them as a generation passing through, the caretakers during this time of change. In later seasons, as the series got more bogged down in the petty day-to-day drama of the Granthams, this sense faded into the background. But the movie gets back to that feeling, and it is all the better for it.
The film also understands implicitly what audiences want to see. The plot is summed up in the trailer: "The King and Queen are coming to Downton." And, in terms of what *actually* happens, that's it. King George V (Simon Jones) and his wife, Queen Mary (Geraldine James), are traveling the country. As Kings and Queens of old did before them, they are using the homes of the aristocracy as free Airbnb units. Of course, the place is thrown into an uproar to get ready. But much of the drama is self-inflicted, and therefore easily self-resolved.
The downstairs staff goes nuts with the idea that they will be serving royalty. Lady Mary (Michelle Dockery) worries Thomas Barrow (Robert James-Collier) isn't doing enough. She then insults him by bringing back Carson (Jim Carter) to run the staff. All this, only to discover Thomas had the right of it by waiting for the royal staff to turn up before doing anything. It turns out the royal family travel with entire teams of overlapping servants, run by "The Page of the Backstairs" Mr. Wilson (David Haig). Wilson does not need any of these provincial yokels who work in a minor house touching anything, thank you. In short order not only is Carson displaced but Mrs. Hughes (Phyllis Logan), Mrs. Patmore (Lesley Nicol) and even Daisy (Sophie McShera) too. At least Barrow's temporary leave gives him time to hang out with the under-valet, Ellis. Ellis shows him, and the audience, a world history has forgotten and adds a romance it would deny could exist.
Upstairs doesn't fare much better. Maggie Smith has the most to do as the Dowager Countess. She has a plan to force the family's widowed childless cousin Lady Maud Bagshaw (Imedia Staunton) to name Robert as her heir instead of the maid she has an oddly close relationship with, Lucy Smith (Tuppence Middleton). But she's not the only one with problems. Branson (Alan Leech) is being stalked by someone who acts as if he believes the Irish Republican son-in-law will try and ruin the King's arrival. Lady Mary discovers she hates event planning. When the chairs for the parade turn up after sundown in a rainstorm the night before the event is supposed to happen, she has to set them up while getting soaked. Lady Edith (Laura Carmichael) also has problems, though it would be a spoiler to say why. Meanwhile, they and Lady Grantham (Elizabeth McGovern) are witnesses to Mary, Princess Royal's (Kate Phillips) marital problems to the Earl of Harewood, and her desperate unhappiness.
But in the end, everything is fine, because that's how Downton Abbey works.
Things threaten to happen, but when all is said and done, very little does. That's always been the secret to the show's success. Look at the first season. The Titanic sinks, the Great War begins. But what actually happens to the Crawleys, other than an untimely death, bringing Matthew into the family? Not much. Lady Mary is saved from scandal by the quick work of mother and maid (Joanne Froggatt). Lady Edith fails to snag a husband, Mary and Matthew do not get engaged. Sybil wears pants.
It's this same soothing level of "nothing really happens" that carries the film along, with the audience knowing that in the end, all will be well. Between the happily ever afters, the heading towards happily ever afters, and the willingness to try for happily ever afters, there's not much left to do by the end of the film but have a ball. It is, after all, what we came for.