The third season of the justifiably popular British import “Downton Abbey,” created and written by Julian Fellowes, comes to PBS on Sunday with the first of seven new episodes set in 1920.

It is the dawn of a new age, not only for the residents of Downton Abbey, upstairs as well as downstairs, but for England as well. The Great War is over, and society is changing. Women are getting their hair bobbed and wearing their dresses shorter – the younger ones anyway. Certainly not the Dowager Countess (Maggie Smith).

The war has taken an economic toll on the nation. For generations, the Crawleys depended on their tenant farmers for income, and Lord Grantham (Hugh Bonneville) found a cash infusion by marrying Cora (Elizabeth McGovern), an American heiress.

But at every turn, the old ways are being forced to give way to the new. Eldest daughter Mary (Michelle Dockery) is getting ready to marry third cousin once removed Matthew Crawley (Dan Stevens), but her father is worrying the family may have to sell the abbey itself unless a financial solution can be found.

Perhaps that solution might be found by hitting up Cora’s visiting mother, Martha Levinson (Shirley MacLaine), who swans into the Abbey with her rough-hewn ways and a forthrightness that adds an extra inch to the Dowager’s frequently raised eyebrow.

Just as Lord Grantham battles to keep things from changing above stairs, butler Carson (Jim Carter) struggles to maintain decorum among the household staff. The formidable O’Brien (Siobhan Finneran) is out to undercut footman Thomas (Rob James-Collier) to advance her nephew in the pecking order.

Lady’s maid Anna Bates (Joanne Froggatt) vows to prove that her husband, Bates (Brendan Coyle), was wrongly imprisoned for the murder of his first wife and continues to see the good in everyone.

The third season will see changes at every level, with the death of a major character, the adjustments Mary and Matthew have to make to married life, changes in the management of the abbey and its properties, and the arrival of Lady Sybil (Jessica Brown Findlay) and her Irish husband, Tom (Allen Leech), the former chauffeur.

All of this makes for one delight-filled episode after another … in spite of weaknesses in the script.

It’s been easy to overlook other issues, such as the similarities not only between “Downton” and the Robert Altman film “Gosford Park,” whose script was also crafted by Fellowes, but also between “Downton” and “Upstairs Downstairs.” It would be less easy to overlook the flaws in the scripts for season 3 of “Downton,” were it not for the fact that we already know and love the major characters.

But if we allow ourselves some distance, Fellowes, particularly in the early episodes of the third season, occasionally advances the plot at the expense of characterization. One major character gets a letter containing information that will have a far-reaching impact, but doesn’t open it until it’s convenient to do so in the company of another key figure in the story.

Really? If you got a letter telling you how your life was going to proceed and change your destiny, would you wait to open it?

On the one hand, all of this smacks of the kind of convenience and coincidence often found in 19th century novels. Yet, in Dickens, for one, when a character needed to announce his or her intentions before following through with them, at least it was within range of our understanding of who the character is. That isn’t always the case in “Downton,” whose characters occasionally behave uncharacteristically.

Our love of the characters makes it more than possible to overlook the sloppiness of the scripts. But it’s because we do know these characters so well that we notice the inconsistencies.

None of this detracts significantly from our enjoyment. Since it’s been renewed for a fourth season, perhaps Fellowes can treat his characters with the respect they’ve earned through how well they were crafted in the first place.