ITV confirmed that the festive episode of the series - which is actually set in the summer season - will focus on Rose's 'coming out' at the Palace, meaning that the family and their servants are staying at their residence in London.
Shirley MacLaine will reprise her role as Cora's mother Martha Levinson, who makes the journey from New York to mark the occasion. She is joined by her playboy son Harold, who is played by highly-anticipated guest star Paul Giamatti.
Elsewhere in the episode, Isobel discovers that she has an admirer, Edith struggles with a "moral dilemma", Carson takes the staff on a day out, and Daisy gets an "unexpected invitation".
But when the Crawleys are implicated in a scandal that threatens to engulf the monarchy, Robert goes to "great lengths" to protect both his own family and the royals.
The episode was filmed in a number of locations in London and the south, with Lancaster House standing in for Buckingham Palace. Shooting also took place at Goldsmith's College, West Wittering beach in Sussex and Berkshire's Basildon Park.
Meanwhile, the special features a number of guest stars, including James Fox, who will appear as Lord Aysgarth - a member of London's high society upon whom Martha and Harold make a big impression.
Elsewhere, Guy Williams will play King George V, while Poppy Drayton, Oliver Dimsdale, Janet Montgomery and Michael Benz will also join the cast for the episode.
Returning to the show are Tom Cullen as Anthony Gillingham, Julian Ovenden as Charles Blake, Daisy Lewis as Sarah Bunting, Patrick Kennedy as Terence Sampson, Tim Drewe and Douglas Reith as Lord Merton.
The Downton Abbey special will air on Christmas Day at 8.30pm on ITV.
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