Downton, which returns to HBO in March 2019, will feature an original musical episode starring Kate Mara and Olivia Colman, according to The Hollywood Reporter.
Mara will direct the episode based on a book penned by David Tennant.
The show is written by David Benioff and D.C.-based writer-producer Daniel Abraham and will be produced by Beniof.
“It’s a great opportunity for us to explore what we have in common as a collective storytelling team,” Abraham said in a statement.
“There’s a lot of great talent in this world.
We are thrilled to be collaborating with such talented people.
And we are grateful to be on board as a co-production partner.”
Mara, Colman and Tennant have been cast as the titular servants and the two-time Emmy winner Colman will play the main character, while Mara has a recurring role as Molly, a housekeeper who is an integral part of the story.
Colman’s co-star, the Emmy winner and Emmy nominee Lena Dunham, will play Molly’s sister, Rosalynn.
The series will also feature recurring appearances from Colman as Molly’s daughter and a recurring cameo from Margo Martindale as Molly herself.
“We are thrilled that Downtons Abbey has landed a new home in HBO and are excited to bring it to the world,” Abraham wrote in the statement.