Well, I suppose they couldn’t sustain the excellence forever. I mean, there’s bound to be the odd weaker episode, right? “Knock Knock”, Doctor Who season 10 episode 3 is certainly that. I have to say it’s the weakest episode we’ve seen so far this season.

This is largely because it doesn’t make much sense. Alien bugs turning people to wood? And back again? Well, I guess it’s no more preposterous than many things we’ve seen, but you need to have some semblance of a rationale, and this had none.

Plus it had less of what has really been making this season great: the Doctor/Bill interaction.

Still, it had an interesting season-arc-related ending, with the Doctor taking Mexican food into the Mysterious Vault to share with whoever is in there. And we now it is a “who:” they were playing the piano. And they eat, presumably.

I think there are two possibilities:

  1. Since The Doctor mentioned regeneration, and we know he’s going to regenerate this season, it’s something to with that. Like a future version of himself, for reasons to be explained.
  2. As I said before, it’s The Master, or Missy, since we saw both the latter and the John Simm version of the former in the season trailer. That would be plausible but weird.
  3. Or, and this occurred to me just tonight: what if it’s Susan? His granddaughter from right back at the beginning? Her photograph was on his desk in the first episode… but that’s just fanciful, and why would he have her in a vault?