The residents of the mansion have been invited to Brancaster Castle in Northumberland by Lady Rose’s father-in-law Lord Sinderby.
Lady Mary and her family may get a nasty shock if the Sinderbys' butler has his way
Alnwick Castle, the Duke of Northumberland's 1,000-year-old seat, has been used to stand in for the fictional estate.
Royal watchers will remember it as the scene of glittering society nuptials last year when the Duke's daughter Lady Melissa Percy wed Prince William's close friend Thomas van Straubenzee. Dubbed the Windsor of the North, the castle also featured as Hogwarts in the Harry Potter franchise.
In Thursday's special we will find out what happens to the Dowager Countess (Maggie Smith)'s potential love affair. In the season finale her craggy Russian beau Prince Kuragin (Rade Sherbedgia) growled seductively: "I want to spend the last years of my life with you".
Unfortunately, his wife the Princess, who was displaced in the Revolution, turns up, played by Jane Lapotaire. What will the Countess make of being at the centre of a love triangle?
Prince Kuragin's wife turns up threatening his relationship with the Dowager Countess
Meanwhile, Lord Sinderby's butler Stowell (Alun Armstrong) is in possession of a deep, dark secret that threatens to ruin the party for his masters.
By Christmas Day itself the action moves back to Downton, where the servants are tackling their own dilemmas. With Anna facing imprisonment for Green's murder Bates decides to take drastic action. Carson and Mrs Hughes are considering a joint business plan or maybe an even closer partnership...
Whatever happens Hugh Bonneville, who is Lord Grantham on screen, says the ending "is very heartwarming. You will have a lump in your throat”.