Spoiler warning for anyone who hasn't yet watched tonight's episode of Doctor Who.
Last week, Doctor Who came back to out TV screens with two new companions, a watery menace stalking them, and a bittersweet ending that saw Bill Potts inducted into the TARDIS gang after losing her crush, Heather. Kickstarting a new season, the series put itself on the path of getting back into the swing of things, and with that comes defining a general objective for the course of the 12 to 13 episodes we'll be enjoying. This year, three words sum up what it's all going to come down to: protect the vault. Now three more words: what's the deal?
In "The Pilot," we saw that The Doctor and Nardole had been holed up at an Earthbound college, protecting some sort of mysterious vault, and we learned a little more about that particular vessel tonight, as that vault is something that The Doctor had promised an unknown third party he'd look after. Its importance is such that not only did he mention it during tonight's episode, "Smile," but Nardole mentioned another detail about the apparently sacred charge that The Doctor has entrusted himself with, that he's made an oath not to leave Earth unless he absolutely has to.
Of course, Doctor Who is made on forming and breaking oaths, so it's only expected that the Doctor go against Nardole's advice that he stay put on campus. Still, it seems that after losing Clara and supposedly closing the story arc of Professor River Song, The Doctor stayed on the campus of St. Luke's University for decades -- all in the name of protecting this vault. However, Bill's curiosity about life, the universe, and everything is the sort of thing that makes The Doctor take up a new companion, and set off into the void of time and space for adventure. Still, this vault is apparently important enough that not only is it hidden away in a college, but it's been mentioned yet again in the context of the show.
So it seems that the story of "The Vault" could, and likely will, be the endgame of the season, which leaves us questioning just what's in there. It's not the first time a vault has played into Doctor Who's storyline, as keeping secrets and hiding weapons both seem to be a recurring themes of our resident Time Lord's world-saving strategy. But while we don't know what is in the vault, we think we might know who he swore his oath to.
If you look at The Doctor's desk in "The Pilot," there are two photos he continually talks to, and they're of the two most important people in his life: River, his most recent wife, and Susan, his granddaughter from the earliest era of the show's history. Seeing as "The Husbands of River Song" is probably still fresh in his mind, The Doctor may have specifically promised his beloved that he'd protect the vault. However, considering the rumors that we may see the First Doctor yet again at Christmas, there might be a chance that Steven Moffat and might be tying up his tenure as Doctor Who's showrunner with a trip to the vault of series history, finally resolving what happened with The Doctor's granddaughter and former companion.
No matter who he swore his oath to, though, there's some important stuff in this vault he's protecting. It may be the secrets to the universe, or even to his own personal history. Whatever it is, he'd better hope he and Bill wind up back home before it's left unattended for too long. It certainly doesn't look like that'll be happening next week, as The Doctor and Bill are about to encounter what looks like a London that's in the midst of an ice age.