In all the excitement of Doctor Who series 10 thus far, you’d be forgiven for forgetting that it’s also introduced a compelling new mystery to the show which seems to be being gradually uncovered week by week – and tonight’s episode Smile was no exception.
 
Of course, when we say “mystery” we’re specifically referring to the enigmatic vault guarded by the Doctor (Peter Capaldi) and Nardole (Matt Lucas) at St Luke’s University, which they’ve apparently been watching over for around 70 years. Based on comments made by the Doctor in the series’ first episode it seems it must contain an object or being of great power, something the Doctor says he “doesn’t want anyone being too curious about”.
 
It also appears that guarding the vault is a mission the Doctor has specifically undertaken for someone else, with the Time Lord and Nardole alluding to “promises” he’s made that have kept him on Earth for so long in the series’ first episode.
 
"I’m here for a reason,” the Doctor says. “I am in disguise. I have promises to keep. No-one can know about me.”
 
Second episode Smile, meanwhile, goes into even more detail about the Doctor’s promise, while also laying out the terms of the Doctor’s forced exile on Earth.
 
“Your oath sir,” Nardole told the Time Lord. “You’re not supposed to go off-world unless it’s an emergency.”
 
Later, the Doctor explains to Bill in rough terms exactly how this oath came about, as well as its peculiar restrictions.
 
“A long time ago, a thing happened,” he says “As a result of the thing, I made a promise. As a result of the promise, I have to stay on Earth. So.”
 
Clear as mud. But at the moment we still have no idea what’s within the vault, with the Doctor electing to travel with Bill for a while and hope the vault's alarms alert him to any trouble. So what’s contained inside it? Who asked the Doctor to guard it? And what could possibly be so important to keep the universe’s saviour Earthbound for such a long period of time?

At the moment, the only clue as to the vault’s contents are the vaguely Gallifreyan-looking symbols seen on its door (above), which could suggest some piece of technology from the Doctor’s home world is the MacGuffin hidden away in the heart of Bristol. 
 
Alternatively, given the already-confirmed return of John Simm’s fellow Time Lord The Master to the series, could it be that it’s something related to him (or his later incarnation Missy, played by Michelle Gomez who’s also back this year)? It could explain the need for Gallifreyan technology, and the “oath” that the Doctor’s made could have been to his own people. 
 
Hell, it could even be the Master or Missy themselves locked in there – after all, didn’t David Tennant’s Tenth Doctor once proclaim his plans to “keep” the Master prisoner in his Tardis in 2007 episode Last of the Time Lords?
 
Then again, it could be that it’s some other completely new deadly creature or device that the Doctor and Nardole are keeping safe for an unknown third party, that we’ll learn the truth of in episodes to come.
 
It certainly seems likely that the Doctor’s decision to travel again will come back to haunt him in future episodes, with the man himself noting that he might be being slightly irresponsible by taking Bill travelling in the below dialogue from episode 2.
 
Bill: Well you’re not guarding the vault right now.
 
The Doctor: Yes I am. I have a time machine, I can be back before we left.
 
Bill: But what if you get lost, or stuck or something?
 
The Doctor: I’ve thought about that.
 
Bill: And?
 
The Doctor: Well, it would be a worry, so best not to dwell on it.
 
The Doctor took his promise seriously enough to stay put for the best part of a century – so what will happen now that he’s broken it?
 
Doctor Who continues on BBC1 next Saturday at 7:20pm