In a new 30-second-clip, the twelfth Doctor (played by Peter Capaldi) can be seen coming face-to-face with the first ever Time Lord (David Bradley) as they try to fight off the mysterious Glass People.

While the trailer focusses largely on the Gallifreyan's adventure together, it also hints at a devastating plot twist.

As Peter's Doctor prepares to take on another formidable army, his companion Bill Potts (Pearl Mackie) issues him a warning. 

Standing at the door of the TARDIS, she instructs him: "Come back alive," to which he replies: "Be here when I do."

DOCTOR WHO CHRISTMAS SPECIAL 2017: RELEASE DATE, CAST, TRAILER, PLOT

Fans already know that the Time Lord will regenerate in the Christmas special as Jodie Whittaker prepares to take on the role as the thirteenth Doctor, but could their exchange hint at an emotional farewell for Bill?

Pearl is also backing down as the Doctor's companion but bosses have remained tight-lipped about her departure - should viewers prepare for a shock death?

Regardless, fans of the BBC sci-fi series should be ready for a dramatic Christmas special, with the synopsis teasing: "Two Doctors stranded in an arctic snowscape, refusing to face regeneration.

"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."

Ending on a mysterious note, it remarks: "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…"

Doctor Who, Twice Upon A Time will air on Christmas Day at 5.30pm on BBC One.