(Contains spoilers for the most recent season.)
It’s been known for a while that the upcoming Doctor Who Christmas special “The Time of the Doctor” will be Matt Smith’s last appearance and will pass the torch to Peter Capaldi. This suggests there will be some variation of the classic “regeneration” scene where the incumbent incarnation of the Doctor “dies” and is replaced by the new one.
But there’s a wrinkle: showrunner Steven Moffat recently said that the Doctor as played by Matt Smith is the 13th Doctor. Why is this important? Because in Doctor Who lore, a Time Lord is only supposed to get 13 incarnations (i.e., 12 regenerations). This means that, in theory, the 13th Doctor would be the last, period. No more regenerations.
(But, wait, you say: I thought Matt Smith was the 11th Doctor? Well, apparently John Hurt’s “War” Doctor featured in the 50th anniversary special “counts” as one of the Doctor’s regenerations. And in the 2008 episodes “Stolen Earth” and “Journey’s End,” David Tennant’s Doctor begins a regeneration after being critically wounded, but channels the energy that would’ve completed the regeneration into his severed hand. Based on Moffat’s comments, this one counts too. So, that adds up to 12 regenerations.)
Which brings us to my theory. One other thing we know about the Christmas special is that it will have something to do with the planet “Trenzalore”:
Trenzalore, as seen in last season’s finale “The Name of the Doctor,” is supposed to be where the Doctor dies once and for all. But instead of leaving a corpse, the dead Doctor left a rip in space and time that allowed one of his enemies to enter his “time stream” and rewrite his entire history. The Doctor was only saved when his companion Clara jumped into the time stream and undid the damage.
But when the Doctor and Clara spot Hurt’s character, a version of himself the Doctor has vigorously suppressed, Clara says that she never saw him but had seen all of the other Doctors, “eleven faces.”
This implies, or so it seems, that any incarnations of the Doctor after Matt Smith’s 11th/13th would have to post-date the Doctor’s “death” at Trenzalore. Otherwise, Clara would’ve seen them during her sojourn into the Doctor’s time-stream.
My theory, then, is that the Christmas special will involve the Doctor’s seemingly final death at Trenzalore, but that through some timey-wimey business a new Doctor (Capaldi’s) will emerge from the ashes. If that’s right, it’ll be exciting to see what, if anything, such a more radical break in continuity will imply for the character.
Of course, I could be completely wrong about this. . .