NEWBURY, ENGLAND — “I just want to get over it,” Lady Mary said. “I mean, over him. I mean, over with. What do I mean?” She burst out laughing and turned back into the actress Michelle Dockery, who plays Lady Mary on “Downton Abbey.” Allen Leech, who plays her brother-in-law, Tom Branson, stepped in to help, “She wants to get it over with.”

It was May at Highclere Castle, the mansion that serves as the set for the titular building, and the cast was halfway through the six-month shoot for the show’s final season. When Season 6, which has already aired in Britain, begins Sunday on “Masterpiece” on PBS, it will be over for Lord and Lady Grantham (Hugh Bonneville and Elizabeth McGovern), their daughters, relations, servants and the love affairs, intrigues, criminal trials, deaths, marriages, withering asides and maintenance of etiquette that have defined “Downton Abbey” since it was first broadcast on ITV in Britain in September 2010.

“I love the way it’s all unraveling,” Ms. Dockery said. “There is a feeling of people moving on with their lives and the sense of the aristocracy as a kind of endangered species. But Mary is trying to keep it going; she doesn’t really like change.”