8 questions we have about the new Doctor Who Christmas special trailer

1. How did they do THAT?

The most thrilling point of the trailer comes at the very beginning: as William Hartnell morphs in front of our very eyes into David Bradley, who will be reprising his role of the First Doctor from An Adventure in Space and Time. It's a staggering technical feat — a wonderful manipulation of old footage from 1966 serial The Tenth Planet. Also, it's a great way of articulating just how perfect Bradley is for this role. The transition from Hartnell to him is practically seamless.

2. Will Twice Upon a Time cross-over with The Tenth Planet?

The Tenth Planet was William Hartnell's last serial before he passed the role on to Patrick Troughton. It's set in a base in the South Pole and is — fittingly — the debut of the Mondasian Cybermen. Based upon the trailer — the opening speech is from The Tenth Planet; it features Polly; the First Doctor is regenerating — it's safe to assume that this is the snowy location the Twelfth Doctor landed on at the end of The Doctor Falls.

How heavily it'll cross over, however, is uncertain. It's more likely that the special will begin during The Tenth Planet and — due to time freezing — end there. In other words, don't expect Trials and Tribble-ations.

3. Who plays Polly?

A surprise for fans of classic Doctor Who is the appearance of Polly, one of the First Doctor's companions. In the 60s she was played by Anneke Wills, but it's not been officially announced who is recreating her youth. Nor if we'll be getting an appearance from her co-companion Ben, originally played by Michael Craze.

4. Will Twice Upon a Time retcon The Tenth Planet?

Speaking of Polly, her appearance is interesting for another reason: within the same shot, the First Doctor is regenerating.

Regeneration was a fairly mysterious thing in The Tenth Planet — with the Doctor suddenly falling ill, collapsing and missing the serial's entire third episode, before returning to regenerate in its final fourth. This was mainly due to William Hartnell's real life health problems, which meant re-writing him out of the story. But — and this is purely speculation — it could also prove an opportunity many years later to explore that period the First Doctor was meant to be resting. Could this be when he popped outside? Where we — and the Twelfth Doctor — overheard him saying how he would not change? Could this frozen moment be when the Doctor first came to terms with his mortality?

5. What has gone wrong with time?

The main gist of Twice Upon a Time seems to be that time has gone wrong somehow; that it's stopped and the two Doctors are, "trapped within a single moment." This is not the first time something like this has happened, of course — The Wedding of River Song, for instance, saw all of time happening at once — and the cause is usually some sort of anomaly. Could this be the meeting of the First and Twelfth Doctor? It's unlikely — after all, this is not the first time this has happened.

6. Who is Mark Gatiss playing?

Speaking of the character, Gatiss revealed earlier to Empire that The Captain had a, "resonant echo in the whole Doctor Who story," and that it was, "a privilege, an honour and a thrill" to play him.

All of which suggests that there's more to 'The Captain' than meets the eye. So, who is he? His Time Lord-esque title doesn't offer many clues. But the fact that he's a World War I captain — and seemingly linked to the show's past — at leasts opens up the possibility that he might be linked to something like the War Games, the 1969 Patrick Troughton serial that saw World War I soldiers kidnapped from their battlefields. Or he could even be a young version of Brigadier Lethbridge Stewart — or at least a relative? The Christmas special does, after all, feature the First Doctor — why not go the whole Classic Who hog?

7. How has Bill come back?

The last time we saw Mackie’s character she’d been saved from life as a Cyberman by her old flame Heather, who transformed Bill into an interdimensional being like herself with the promise that she could also return her to human form if she so wished. So which one do we see in this trailer? Normal human Bill or space-travelling demigod Bill? Either way, it's a thrill to see the Tardis twosome reunited no matter how they manage it.

8. Why is it called Twice Upon a Time?

As obvious speculation is that it's referring to the story starring two Doctors. After all, the episode title The Two Doctors has already been taken by the 1985 serial starring Colin Baker and Patrick Troughton. The riff on the old phrase 'once upon a time' could also be a nod to showrunner Steven Moffat's philosophy that Doctor Who, at its heart, is a fairy tale. But of course, it's probably a reference to something completely unexpected and clever. It always is.

Doctor Who returns to BBC1 with Twice Upon a Time on Christmas Day