It is the summer season, and the family and servants are at Grantham House, their London residence, preparing for the busy social season — but they are embroiled in a scandal that threatens to engulf the British monarchy.
As Lady Rose is presented to King George V and his wife Queen Mary by her aunt Cora, Robert is forced to act to protect the royal family, and his own.
Fans can also look forward to the return of Cora's American mother, Martha Levinson, played by Hollywood legend Shirley MacLaine, and her playboy brother Harold (Paul Giamatti), fresh from his recent brush with the law.
Other confirmed guest stars include James Fox as Lord Aysgarth, Guy Williams as King George V and Oliver Dimsdale as the Prince of Wales. Tom Cullen and Julian Ovenden, who played two of Lady Mary's love interests in the last series, are also set to return.
The feature-length episode will be shows on ITV on Christmas Day at 8.30pm. It will include all the cast from the fourth series, including Dame Maggie Smith, Hugh Bonneville, Michelle Dockery, Joanne Froggatt and Brendan Coyle.
Filming locations for the 2013 special include Lancaster House, representing Buckingham Palace, West Wittering beach in West Sussex and Basildon Park in Berkshire, which is standing in for the Crawleys' London residence.
The finale of series four was viewed by 10.5 million people. ITV announced in November that a fifth series has been commissioned.