Maggie Smith in “Downton Abbey.” (Nick Briggs/Carnival Film & Television Limited 2015 for Masterpiece)

All along, “Downton Abbey” has busily issued fond goodbyes to the way things used to be a century or more ago; to the very end, it’s a show about the difficult transition and eventual extinction of a particular strain of the upper class, luxuriating in a lifestyle that Anglophilic viewers and fans of the show hold dear. Now, in its sixth and final season, which begins Sunday night on PBS’s “Masterpiece,” the hardest truths of the 20th century are no longer abstract or optimistically shrugged aside. Downton is going down.

Mr. Barrow (Rob James-Collier), the snaky under-butler, has been all but commanded to find employment elsewhere as the Grantham estate begins to tentatively curtail its staff expenses. He sees only a dismal, unfocused projection of what lies ahead.

“I can’t see the future,” he says, glumly. “But then I suppose none of us can.”

He could be speaking for the entire house. “Downton Abbey” has always been at its strongest when comparing the past with the rush toward modernity that has shaped its unforgettable characters, whom we first met in 1912 and are now parting company with in 1925. In between was World War I, with its direct and grievous effects on a generation, regardless of class or background, but “Downton Abbey” was better when zeroing in on subtler shifts — hints of gender equality, a blurring between nobility and servitude, and the arrival of everyday technological advances. (This season it’s a refrigerator and a hair dryer that get the stink eye from the house traditionalists.)

So Barrow is looking for a new job, preferably as a butler in a suitable estate where he’ll no longer squirm under the thumb of Downton’s head butler, Mr. Carson (Jim Carter). The want ads lead Barrow to a run-down manse in which a wistful, elderly gentleman rattles around and pines for the Victorian era, when the house played host to visiting dignitaries and royals. “We can’t let them down, you see,” the old man tells Barrow, who can barely hide his horror at the house’s decrepit state. “When the good times return and they all come back, we must be ready. Can’t let our standards slip.” In a perfectly sublime moment, Barrow is the first to see up close that the dreaminess of places like Downton Abbey has passed.

Joanne Froggatt as Anna Bates and Brendan Coyle as Mr. Bates. (Nick Briggs/Carnival Film & Television Limited 2015 for Masterpiece)

Jim Carter as Mr. Carson and Phyllis Logan as Mrs. Hughes. (Nick Briggs/Carnival Film & Television Limited 2015 for Masterpiece)

With a gentle caution about spoilers ahead (Season 6 has already aired in the United Kingdom), it seems safe enough to reveal that these final episodes of “Downton Abbey” are among the show’s best since the first season — and they’ll reassure those hoping for the happiest possible endings for nearly every character. Even the aforementioned Barrow is somewhat redeemed of his sins. The final episode, which will air here on March 6, got a hearty thumbs-up from fans and recappers across the pond, who watched it on Christmas Day and report no tragic or unexpected swerves at the last minute. (To truly love “Downton” is to revel in its utter predictability at this point.)

Creator and writer Julian Fellowes introduces his usual array of minor dust-ups and briefly emotional subplots, but, for the first time in a few seasons, “Downton Abbey” feels purposefully structured and intent on sticking the landing. Scenes that loyal viewers have waited five years to see finally come to pass in the back half of this season, and they’re especially good, requiring the best work yet from the show’s superstar, Maggie Smith (as the Dowager Countess) as well as Michelle Dockery (as Lady Mary) and Laura Carmichael (as Lady Edith).

Meanwhile, Mr. Carson and Mrs. Hughes (Phyllis Logan), who were engaged last year, fuss over wedding plans and the parameters of their impending, um, intimacies. Isobel Crawley (Penelope Wilton) finds herself in an unenviable showdown with the Dowager Countess over the fate of the village hospital; Mr. Bates and Anna (Brendan Coyle and Joanne Froggatt), free of their respective rap sheets, are trying to have a baby.

As for set pieces and memorable scenes, there are thrilling auto races and an unfortunate dinner table incident that’s straight out of the first “Alien” movie. Mostly it’s the usual lightly soaped “Downton” inanities until the fifth or six episode, when Fellowes realizes that there’s not much time left to dawdle and that every sentence is going to have to end with a period. Edith has come into her own as a magazine publisher, but will her secret love child (any “Downton”-related drinking games should involve the word “Marigold”) come between her chances of romance with Bertie Pelham (Harry Hadden-Paton)? Will Mary find love with Henry Talbot (“The Good Wife’s” Matthew Goode), despite the fact that he’s the upper-crust equivalent of a race-car obsessed slacker?

The final season of “Downton Abbey” begins Jan. 3 on PBS. (Nick Briggs/Carnival Film & Television Limited 2015 for Masterpiece)

Perhaps my favorite moment of this final season comes with a glimpse into the house’s future as a tourist trap (reflecting the present-day mission of Highclere Castle, where the show has been filmed). To help raise funds for charity, the Crawleys open Downton for a day to commoners, who take in its splendor and ask all sorts of questions about the house and its history, to which the family doesn’t always have the answer. The Earl of Grantham (Hugh Bonneville) has an unlikely encounter with a young boy who has peeled off from the official tour and wants to know why anyone would need or want to live in such a big house.

“Downton Abbey” may strike some as an unbearably fussy and silly affair, but it has also fully and richly answered that little boy’s question: They lived that way out of a sense of duty. And while it lasted, wasn’t it grand?

Masterpiece: Downton Abbey (one hour) returns Sunday at 9 p.m. on PBS stations; continues through Feb. 21. Series finale airs March 6.