How will Pearl Mackie return to Doctor Who this Christmas?

However, there is still a bit of a mystery surrounding Bill’s reappearance. The last time we saw her, she had been transferred from her Mondasian Cybermen body into a new, spacey-wacey form by old flame Heather (Stephanie Hyam), and told that she was essentially now an immortal superbeing who could travel the cosmos freely.

“So I'm like you now. I'm not human any more,” Bill says at this point in the episode.

“I can make you human again,” Heather responds. “It's all just atoms. You can rearrange them any way you like. I can put you back home, you can make chips, and live your life, or you can come with me. It's up to you, Bill.”

So here’s our big question: will the returning Bill be human again? It seems like the most likely option – there’s no word of Hyam returning for the special, and if Bill is a major part of the story, having her as an immortal being who can change reality would rob the adventure of a little peril. Why be worried about danger when she could probably just bring people back from the dead anyway?

On the other hand, it seems impossible that this story could include ordinary, human Bill. While we don’t know too much about Twice Upon a Time, we do know that it involves Peter Capaldi’s Twelfth Doctor teaming up with his first incarnation (David Bradley taking over from late original actor William Hartnell) in the South Pole in 1986 (the original setting of the First Doctor’s last serial the Tenth Planet, where The Doctor Falls concluded), and then at some point heading back to the First World War.

Later, the pair seem to travel through time (and possibly space) again given that we see Gatiss’s WWI Captain in the Tardis – so given all this time-trotting, what chance would normal human Bill have at catching up with them? Surely an intergalactic, atom-crafting superbeing would have better luck tracking down the Doctor (as the festive trailer, below, seems to imply she’s been trying to do).

Sure, it’s possible that the Time Lord twosome could catch up with her in present-day Earth, but so far we’ve had no indication that the episode will visit the here and now so it seems more likely that she’s catching up with them somewhere else in space and time – something she’d struggle to do if she was an ordinary human.

Plus, we have to think about the other looming mystery surrounding Bill’s return – the fact that this episode will definitively mark her exit from the series, at least according to Mackie herself.

“No, that’s it – Christmas special, last chance to see Bill,” Mackie said at San Diego Comic-con last month. “But, I mean, hey, it’s Doctor Who, so never say never!”

With this knowledge, to answer the question about Bill’s humanity we have to think what sort of exit Pearl Mackie’s likely to be given. It seems unlikely that she’d be killed off in a Christmas episode, and the Doctor just “dropping her back” home right before his regeneration (when he’d need her most) would seem a little anticlimactic. In the episode trailer Bill seems overjoyed to see her old tutor – would she really abandon him so easily?

Bill reunites with the Doctor in Twice Upon a Time

So having Bill be something more than human again might be the most satisfying exit she could be given. Maybe what we can really expect is more of a grey area for Bill – returned to human form by Heather to find the Doctor, then “picked up” by her alien engine oil pal (yes, this is a weird show) to continue their intergalactic adventures by the episode’s end, with the Twelfth Doctor’s blessing as he makes his own final journey.

But of course, we could be well off-base with all this. For now, we’ll have to wait and see what form Bill Potts takes for her final tutorial with the Doctor – more details are likely to emerge between now and December – and we can only hope she’s given the emotionally satisfying, narratively consistent and suitably festive send-off she deserves.

High expectations? Well, we’re only human.

Doctor Who returns to BBC1 this Christmas