Its future has hung in the balance due to Lord and Lady Grantham's lack of male heirs (series one), and its routine has been disrupted when it became a hospital during the First World War (series two).
Now, with the war over and Lady Mary betrothed to her father's closest male relative, Matthew Crawley, one might expect each resident to be happy, not least because there's a wedding to look forward to.
But, as the third series of the Bafta-winning show kicks off, it's not to be. Lord Grantham (played by Hugh Bonneville) has jeopardised the fate of the estate by gambling nearly all of his wife's fortune on ill-advised investments.
"The possibility they might have to downsize is a reality," says Bonneville. "That's why he's emotional – it's been his life's work, to hand on the estate to the next generation."
To compound matters, the social order of the house is threatened by the return of Lady Sybil and her husband, Downton's former chauffeur Branson.
Over from Branson's native Ireland for Mary's wedding, the pair are expecting a child and pose a dilemma to Downton's servants, who now have to serve someone they used to work alongside.
Steadfast butler Carson is the most ill at ease with the new arrangements.
Jim Carter, who plays the orderly chief of staff, says: "Carson is a constant. He's like the house; he's always there and never changes."
It's not easy for Sybil and Branson either. Allen Leech, who plays Branson, said the scene in which he dines with his former employers for the first time was a minefield.
"I sat down and Maggie Smith [who plays Dowager Countess Violet] was opposite me and I thought, 'I shouldn't be here!' The others have been schooled in the table etiquette involved. I had no idea!"
Also appearing at Downton for Matthew and Mary's wedding is Martha Levinson, Cora's mother, played by Hollywood legend Shirley MacLaine.
Bonneville says: "The words 'the mother-in-law is coming to stay' have a certain reaction from men all over the world, and Robert is no exception."
Martha hopes she can shake things up, leading to classic confrontations between Martha and Violet.
"She is extremely outspoken," says Oscar-winning actress MacLaine. "Her basic role is to plead with the Dowager Countess to wrest herself, if possible, away from tradition."
The first meeting between Dame Maggie and MacLaine was momentous, reveals Bonneville.
"It was like Stanley meeting Livingstone," he says. "Shirley lifted Maggie off the ground and said, 'My God! Maggie!"'
One person pleased to see Martha is Lady Edith, who is also thirsty for change after experiencing some freedom during the war.
Laura Carmichael, who plays Edith, says: "The young girls can't go back to how it was before. They've experienced a job and some independence, so going back to dinners and riding is not going to be enough."
Elizabeth McGovern, who plays Cora, agrees. "The war has been the cataclysm and now we're seeing the ripple effect."
For Lady Edith, this means being more forthright with her pursuit of Sir Anthony.
"She doesn't bat an eyelid that people think she's batting out of her league. I think that's lovely to watch," says Carmichael.
The first episode sees the culmination of the 'will they/won't they?' storyline between Matthew and Mary, and Dan Stevens, who plays Matthew, reports the wedding does go ahead.
"We would have been hard pushed to stretch the storyline any longer," he says, laughing. "But there's still the issue of will they or won't they be happy? Will they or won't they have children."
Another relationship at the heart of Downton Abbey is that between valet Bates, now in prison, and head housemaid Anna.
Brendan Coyle says: "It was fittingly bleak."
The only light amid the darkness is the love of Anna, who stays true.
Previous partners in crime, valet Thomas and lady's maid O'Brien see their friendship end – and kitchen maid Daisy isn't happy either.
Actress Sophie McShera says: "They bring in a new scullery maid and Daisy is livid she's been replaced with a younger, more beautiful model."
l Downton Abbey, ITV1 on Sunday, September 16