It's 1923 at 'Downton Abbey', and Rose is to be 'outed', in the London 1920s fashion - the opportunity for Julian Fellowes to pick up his pen with a particularly jovial flourish, and put together all the usual suspects, together with some royalty, a conniving schemer and - most disturbingly of all - Americans.
That these New World interlopers came in the high-pedigree form of Shirley MacLaine and Paul Giamatti only added to the fun. As brother Harold, Giamatti especially added an understated but effective presence to proceedings, as a playboy not easily persuaded to give up his hedonistic ways.
Fellowes has form in pitting the family narrative against both social history and real-life luminaries - Downton has already played host to legendary diva Dame Nellie Melba - and this 2-hour special saw him expand his canvas to include the pleasure-seeking Prince of Wales.
The Crawleys got involved in a strange escapade, helping him to avoid a scandal with his real-life ladyfriend Mrs Dudley-Ward, courtesy of a completely invented card game and Mr Bates' 'street' skills, i.e. forgery and pocket-picking. This provided the double bonus of some neat in-jokes - 'when that Bertie gets himself into trouble next time, it won't be the Crawley's fault, tee hee' - and reminding us that Mr Bates was once an interesting fella, and could be again.
Less fun was Lady Mary's interminable dilemma over which earnest-browed bore she should encourage, Mr Blake or Mr Gillingham. Guess what? In the end, neither. Well, there is Series 5 to think about, after all. Which something tells me will include some reference to Mr Blake's sizeable holding in Ulster...
Back in Yorkshire, it was all a bit quiet by contrast. It was pretty obvious Branson had plans a-foot, which turned out to be eating at the pub, bumping into Miss Bunting and taking her on a tour of the house, which was pretty awkward BEFORE they bumped, inevitably, into a lurking Mr Barrow - a vindictive valet sorely underused in this series, Christmas special included.
Home turf still afforded the best laugh of the night, however, when Lord Merton paid a visit to Mrs Crawley. Not only did he tell her, with a straight face - "I'm on my way to dine with the Scroops" - WHO? - but then, out of the blue, Mrs Crawley went all continental on him - "I'm much more serieuse," she told him, with an equally straight face. Obviously a match made in heaven. Dear me, Lord Fellowes must have had fun writing that bit.