Lady Mary makes one more most-unladylike sexual mistake, “Mum-MAH” dishes up one more serving of proto-feminist American common sense and one last “season” in London town is experienced for “Downton Abbey: The Grand Finale” of the highbrow British/PBS soap opera that has also produced three movies.

It’s a starchy send-off filled with stiff dialogue and so much fan service that it fairly drowns in characters and new versions of events and scenes that we’ve watched played-out in other forms over the long history of this period piece franchise. All those episodes and movies, it can’t help but play as repetitive and recycled.

How many more affairs, dressing room confidences exchanged with the maids and strolls around the grounds can one endure?

But then a pretty fair proximation of the “Downton” era’s greatest wit — the playwright, actor and gay bon vivant Noël Coward –– shows up. And damned if Downton and those who made it don’t make a graceful, warm bon voyage into the cinematic sunset.

I’d always suspected that perfect bookend to this saga would be to end it as the house is taken over for the all-consuming struggle that was Britain facing Nazi Germany in World War II. The series began on the eve of WWI after all, and the estate and the poshes who live there and the plebes who served them did their bit as The Great War errupted around them. The fact that many “Great Houses” did not come out of the Second World War in the same shape or the same hands, something “Brideshead Revisited” touched on, seemed to suggest the perfect coda.

But creator Julian Fellowes, who used his script for Robert Altman’s “Gosford Park” as a dry run for “Downton” — knows best. And he begins this outing in style, with a “season” at Grantham House, the family’s townhome in London, where the East End features a play by Coward (Arty Froushan, terrific) starring dashing old actor friend of the family Guy Dexter (Dominic West), whose backstage dresser and “pal” is Mr. Barrow (Robert James-Collier), once a closeted valet at Downton.

Lord Grantham’s beloved mother (Maggie Smith) died in the last film. And now American-born Lady Grantham (Elizabeth McGovern) has lost her mother (played by Shirley MacLaine, remember) and her affairs are in a mess.

Because Lady Grantham left her businessman/brother (Paul Giamatti) in charge. The Wall Street crash almost wiped them out, and he took on an advisor, Gus Sambrook (Alessandro Nivola), who is tall, dark and sketchy.

The Lord may be putting the Abbey in the hands of their daughter, Lady Mary (Michelle Dockery). But she’s not above being plied with the latest cocktails from America by the wily and beguiling Gus.

Yes, she winds up in bed with him, which considering word of her divorce has just scandalized her and put her on the “shunning” list of London and Yorkshire society, isn’t a smart move. And yes, that’s exactly how this saga began, 16 years ago, with Lady Mary bedding a houseguest and trying to keep that a secret when he winds up dead the morning after.

Gus, being American, doesn’t have the good manners to die and be less of a problem.

The Lord may or may not relinquish control of the Abbey and its now-turning-over staff. Mr. Carson (the regal Jim Carter) is retiring, as is the earthy, old school cook, Mrs. Patmore (Lesley Nicol). The increasingly confident and self-assured Daisy (Sophie McShera) is taking over for her, and her refusal to be overly impressed by her “betters” makes egalitarian in-law Lady Merton (Penelope Wilton) appoint her and the retiring Carson to the committee that runs the local fair.

They’ll do battle with the formidably snobby, Lady Mary-shunning Sir Hector (Simon Russell Beale).

With dicey finances, “improvements” needed for the cottages on the property and more “changes” than one old Lord and Lady can handle on their own, it’s nice that help is never more than one kindhearted relative away.

The widowed Tom (Allen Leech), once a servant, now a socialist rolling in just-sold-my-business cash, will pitch in. The staff will spread the right rumors and “scandal,” “cash poor” or not, all will be made right by this nice world of nice people, not all of whom were “nice” on the series.

The vast clutter of characters to be served (Kevin Doyle plays Mr. Mosely, the ex-footman who married into the staff and became a name-dropping screenwriter, Joan Froggatt and Brendan Coyle are the Bates, staffers who married and face the prospect of being split up as households move, etc.) and Fellowes’ tendency to rub the rough edges off characters whom fans invested in makes this the blandest of the “Downton” films.

“I AM English,” Lord Grantham huffs unnecessarily. “And I thank heaven for it!”

The dialogue is page after page of bland variations of “It’s your time, now” and “the changes we’ve all seen.”

The class war is watered down, with even the poshes embracing the “new world” where servants have rights and agency and can be recognized for their intelligence, skill and ambition, if not their “breeding.”

The earliest scenes, with a digitally-recreated 1930 London, are the prettiest. The summery settings are lovely as ever. But for a period piece, this one looks visually flat. Director Simon Curtis (“My Week with Marilyn,” “Goodbye, Christopher Robin”) is content to make the costume changes and parades of vintage motorcars run on time and let the players play what they’ve been playing for years.

But vintage recordings of Coward singing adorn the soundtrack. And when the theatre folk take center stage in the third act, things liven up and even the overdose of “nice” goes down easily.

Froushan’s Coward is a twinkling, singing suggestion of the real thing, and West takes the spotlight like only an actor playing an actor can.

“Downton Abbey: A New Era” scored its most sentimental points by saying goodbye to Maggie Smith. “A Grand Finale” may not be all that grand, but it more or less checks off the boxes in allowing fans to revel in this world one last time and bid the great house and great cast bon voyage, even if the low-stakes/no-stakes send-off isn’t all it might have been.

I mean, it’s not like “There’s a WAR on,” you know.

Rating: PG

Cast: Michelle Dockery, Hugh Bonneville, Elizabeth McGovern, Phyllis Logan, Laura Carmichael, Joan Froggatt, Sophie McShera, Penelope Wilton, Paul Giamatti, Robert James-Collier, Alessandro Nivola, Dominic West, Kevin Doyle, Arty Froushan and Jim Carter.

Credits: Directed by Simon Curtis, scripted by Julian Fellowes, based on the TV series and characters he created. A Focus Features release.

Running time: 2:03