“Regeneration is coming.”

So intones the latest teaser for the upcoming “Doctor Who” Christmas special, which features the 12th Doctor (Peter Capaldi) teaming up with the first (“The Strain” star David Bradley, portraying the 1960s-era Time Lord played by the late William Hartnell) before he regenerates into the series’ first-ever female Doctor (Jodie Whittaker, expected to make her debut at the end of the episode).

In addition, Whovians can also bid farewell to Capaldi — exiting the show after three seasons — in a new farewell special, with the announcement coming along with the release of the new teaser (which can be seen above).

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Featuring narration by Colin McFarlane (“The Dark Knight”), the farewell show includes archival footage — including Capaldi’s first script read-through — and interviews with departing showrunner Steven Moffat, who’ll reveal his favourite episodes from the Capaldi era as well as a look back at some of the best and worst moments.

“The magical final chapter of the Twelfth Doctor’s (Peter Capaldi) journey sees the Time Lord team up with his former self, the first ever Doctor (David Bradley, ‘Harry Potter’, ‘Game of Thrones’) and a returning Bill Potts (Pearl Mackie), for one last adventure,” reads the BBC synopsis of this year’s Christmas special, titled ‘Twice Upon A Time’.

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“Two Doctors stranded in an Arctic snowscape, refusing to face regeneration,” the synopsis continues. “Enchanted glass people, stealing their victims from frozen time. And a World War One captain destined to die on the battlefield, but taken from the trenches to play his part in the Doctor’s story. An uplifting new tale about the power of hope in humanity’s darkest hours, ‘Twice Upon A Time’ marks the end of an era. But as the Doctor must face his past to decide his future, his journey is only just beginning.”

The “Doctor Who” Christmas special and the farewell tribute special will air on Christmas Day.