WASHINGTON — "Downton Abbey," that phenomenally successful British drama series about a very specific sort of people enduring what amounts to the modern era's original first-world problems, is back on PBS's "Masterpiece Classic" Sunday night for a third season. It is greeted with huzzahs, which are deserved, but it is also met with the law of diminishing returns.

Only a fool could overlook the sinkholes in last season's plots: Along with characters miraculously leaping out of wheelchairs and/or breaking into song, an unwanted fiancee conveniently dropped dead of Spanish flu as "Downton Abbey" verged on soapy self-satire. Creator/writer Julian Fellowes seemed to making it up on the fly, as tragic plot twists were hinted at, fretted over and then strangely vanished. A wounded man, comically covered in mummy bandages, arrived at Downton's makeshift World War I hospice claiming to be the Crawley estate's rightful heir — and then what happened?

Nothing. A whole lot of nothing. It was disheartening to see "Downton Abbey," which so effortlessly charmed us in the first season, fall so flat in the second. Some fans still will not concede this. "I just like watching it," they whine in defense. "I watch it for the costumes, the domestic details, the dowager countess — all of it."

Well, who doesn't? I, too, would probably watch "Downton Abbey" even if it were only about serving dinner each night. (And, mostly, that's what it is. Would it work just as well as a cooking show on the Food Network?)

'Downton Abbey': What to Eat While You're Watching

The good news is that we can all take a measure of comfort in this latest effort. "Downton Abbey" comes back stronger and more muscular this time, with intriguing and shocking new plots that provide a bit of vital momentum and an uncharacteristically wrenching dose of tragedy when two major characters die — which I shan't spoil any further, though goodness knows the Internet already has, once Season 3 began airing in Britain a few months ago and just concluded with a Christmas story that contains a dark development.

Before all that, in a satisfying and brisk opening episode, it's still 1920-ish, pretty much always spring, and Downton is getting ready for the wedding of the century: skeletal cold fish Lady Mary Crawley (Michelle Dockery), the eldest daughter of the Earl and Countess of Grantham, is at last marrying her distant cousin, the heroic and handsome Matthew (Dan Stevens).

A pall has fallen over the house, as palls must, because Mary's father, Robert (Hugh Bonneville), has lost the Crawley family's fortunes to bad investments. In a truly moving scene, Robert confesses this to his wife, Cora, played by Elizabeth McGovern, an actress who often carries "Downton Abbey" squarely on her capable shoulders amid so much silliness and fawning over Maggie Smith's delectable snootiness as Cora's mother-in-law, the Dowager Countess Violet. In private, Cora bravely encourages a humiliated Robert to postpone his despair and enjoy the wedding, even if it's a farewell to the opulence they've known all their lives.

The nuptial buildup involves an almost orgiastic flurry of gorgeous gowns, party prep and menu planning — the show's surest addictive narcotic. The marriage assures the immediate future lineage of the (now-broke) estate and tidies up the show's main love story, getting Mary and Matthew between the sheets at last, but their bedroom patter is more boring than their pajamas. All they talk about is money and infertility.

As antsy "Downton Abbey" fans already know, Cora's wealthy mother, Martha, arrives from the United States for a visit, in the welcome personage of Shirley MacLaine, who has a total ball in this cameo. Martha's brought with her the winds of change, or something like that; in America, you see, women have won the vote, skirts are shorter, jazz is jazzy, formal dinner parties can just as well be a cocktail buffet, and so on, as per "Downton's" Wikipedia-like need to footnote itself with historical touchstones.

In the PBS equivalent of a wrestling smackdown, Marvelous Martha and the Dour Dowager swipe verbal kitty claws at each other here and there, but the scene you're really waiting for — where two similar yet opposite women have a reflective talk about what they've seen in life — never comes.

That's because Fellowes, for all his devotion to the early 20th century, has a true 21st-century attention span. At times, it almost feels as if "Downton Abbey's" stories and dialogue are written one tweet at a time. He doesn't like for any one part of his multi-tracked, upstairs-downstairs saga to drag unnecessarily (or even necessarily) on, and he's not big on payoff.

That wedding we so breathlessly await? It happens, but Fellowes's blunt sense of transition chops it off the minute Mary walks down the aisle, moving forward. (Funerals get the same treatment.) "Downton Abbey" subsists on characters describing for one another what just happened off-camera; news is delivered instead of witnessed. In that sense, it's a series that writes its own recaps and prefers "tell" over "show." And some of Season 2's bad habits remain — introduce a spot of bother (a cancer scare, for example) and then let it pass uneventfully, all that worry for nothing.

That would usually make a scripted drama a chore to enjoy, but "Downton Abbey's" frantic bustle is, I suppose, part of its strange sense of relevance. It's no accident that American audiences fell hard for this vicarious entry into aristocracy amid our own economic woes. It's about the transition from one sense of culture and society to another, in which one person's progress is another person's last gasp. Everyone in Downton Abbey — master or servant, 1 percent or 99 — seems to realize that the game is up, that this way of life cannot last much longer.

Much of the new season focuses on the estate's fiscal cliff: Will Matthew use a surprise inheritance to save the day? Could Martha fork over still more of her fortune to prop up the Crawleys? Could Downton and its village self-sustain with a new agribusiness model? Or should the family shut it all down, sell it off and move to a "smaller" house that they own in the "country"?

If Fellowes is really in a hurry, I'd like to see "Downton Abbey" leap forward a decade or more to show us what the end of that era looked like. (Instead, there are rumors of a "Downton" prequel, going back into the 19th century, when Robert and Cora meet.)

At this rate, we'll never get to see Downton and its occupants during World War II, or later still, in its inevitable fate as a modern-day tourist trap with no permanent residents. It feels as if Fellowes has told as many stories as he can about these particular people in this particular moment, and yet the draw is undeniable. I'm fairly sure we'd watch them just sit with their grief for at least another season or two.

Downton Abbey returns Sunday at 8 p.m. on PBS; check local listings for channel.