The third season of “Downton Abbey,” which airs its finale on Sunday night, began with an event that ends most stories: a wedding. Not just any wedding—the dynastic, sanctioned, semi-incestuous union of Mary Crawley (Michelle Dockery) and her cousin Matthew (Dan Stevens). It’s a happy ending—the star-crossed lovers are together, the Crawley family will keep their home, and the servants will keep their jobs—but it’s not a promising start for a soap opera’s third round. Shorn of sex scandals and fraudulent amnesiac heirs, “Downton” this season has lacked the soapy suspense that used to power it. What remains is essentially conservative: The future will revolve around preserving things, and Downton in particular. So what happens to a show like this, when it loses its propulsive plot engine but keeps on spinning? Is “Downton Abbey” as afraid of change as its characters?

The show has long since disappointed whatever expectations its audience once had of serious social commentary—this is not “Upstairs, Downstairs.” It lacks the earlier show’s darkness, and if once upon a time Julian Fellowes's decision to humanize the downstairs help seemed aimed at making viewers question an aristocratic institution, the show is now fully committed to making us root for Downton. Any notion that the estate is not a benevolent employer gainfully supporting hundreds of people—and an overall social good, if badly managed—is only acknowledged in passing. Back in the first season, Gwen the maid’s departure to be a typist seemed to herald broader horizons for the staff in a changing world. Gwen’s life was hard. We saw her getting up in the cold and struggling; life downstairs was unpleasant. That’s no longer the case. We don’t see the servants rising in the dark, or cleaning, or scrubbing. Instead, they’re waiting at table and doing ladies’ hair and eating together and having tea. Even their rooms seem less drab. When it comes to preserving Downton and the social order it represents, the servants and the family are literally on the same cricket team.

While the show’s characters debate how to adapt to the changing social mores of the early 1920s, the show itself stays fixed. In fact, that’s part of its pleasure, like watching a sitcom: the happy, slightly unhealthy joy of watching the same set of beloved characters share screen time, work through some crises, then return to a point of comparative stasis. Certain formulas delight: Cousin Violet, the Dowager Countess (Maggie Smith) will always say something cutting and witty, O’Brien and Thomas Barrow will always plot with or against each other, Daisy will sulk, Edith will chafe, and Bates will continue to be Job in Downton’s universe (Fellowes is so determined to have Bates suffer that I fear for his wife Anna’s life next season). The show finds its equilibrium when plot becomes sedentary and details come to the fore. If Julian Fellowes were a composer he’d be Pachelbel; themes and variations are his medium, and this is the season of the reprise.

In the absence of real conflict, it’s unsurprising that Fellowes flirts with anticlimax this season like never before: Mrs. Hughes, the kindly housekeeper, might have had cancer but didn’t. Youngest daughter Sybil’s escape from Ireland might have gone wrong but went right. Mary might have been infertile, Edith might have married, and the Crawleys could have lost Downton. None of these events took place, and their aborted status is exactly the point: The show has moved into the subjunctive tense. Fellowes leaves dozens of guns on the mantle unfired, and resorts instead to a surprise axe to the back. The death of Sybil, the third Crawley daughter, is just such an event, and it tests the show’s commitment to stasis. That enormously affecting scene in which Cora (Elizabeth McGovern) stares into the camera as she says goodbye to her daughter Sybil, repeating the words “my baby,” is one of the most disruptive scenes the show has ever had—not because of the death itself, shocking though it was, but because that moment contains the possibility of permanent emotional brokenness, of “Downton” becoming a tragedy. When Cora asks, “Is it ever really over?” we wonder whether this will be a watershed: People have died on “Downton,” but no one has yet been crippled by lasting emotional damage. The question hovers in the air for the length of exactly one episode, during which Cora’s pain renders her unable to forgive her husband Robert (Hugh Bonneville) for trusting a well-heeled doctor and ignoring his daughter’s health. A rare condemnation of aristocracy hangs in the air, and it seems that this is a problem “Downton” will not be able to fix—until Cousin Violet mops up the messiness of grief with a tidy lie.

It works, and “Downton” returns to form: Cora is back to her usual resilient self by the next week. Sunday’s finale, which aired in Britain as a Christmas special, contains one very shocking plot twist (if you’ve managed to avoid the spoilers), but we will have to wait until next year to see what lasting effect it has. “Downton” is basically sadness-proof, and it will always be more Wodehouse than Brontë.