The closing moments of this evening’s Doctor Who Series 10 finale revealed that this year’s Christmas special – which is still unnamed – will be a multi-Doctor adventure featuring the current Doctor (Peter Capaldi) and the First Doctor, as played by An Adventure in Space and Time‘s David Bradley.
The cliffhanger ending saw the Doctor fatally injured and beginning to regenerate. Stepping out of the TARDIS into an arctic landscape, he fought off the glowing regeneration energy and fell to his knees, vowing that this time he would not live on and change into a new Doctor, for some reason. This was declared “ridiculous” by an approaching figure, who mocked the Doctor as he stepped forward through the snow to reveal himself as the very first incarnation of the Time Lord (“The original, you might say”) just before the credits rolled. The story will continue at Christmas.

This will be the first time that the First Doctor has played an active onscreen role in Doctor Who since 1983’s The Five Doctors, when the character returned to the series following his regeneration almost 20 years prior. Doctor Who’s most recent multi-Doctor episode aired in 2013, when The Day of the Doctor marked the series’ 50th anniversary.
Let’s not go all this time without recalling William Hartnell, the actual First Doctor.
Further casting and details for the Doctor Who Christmas special will be revealed in due course. As previously announced, the special will serve as Peter Capaldi’s farewell to the series, and will feature the debut of the next Doctor. And no, I don’t know why the Doctor doesn’t want to regenerate, either.