Doctor Who season 10 has begun, premiering to strong reviews. According to showrunner Steven Moffat, season 10 was meant to serve as a ‘soft reboot’ of the franchise, and that certainly seems to be the case. Pearl Mackie has injected a breath of fresh air into the show as the newest companion, Bill Potts. Watching the new relationship unfold between Bill and Peter Capaldi’s Doctor is wonderfully intriguing, and though Bill is a complete newcomer to the prospect of space and time travel, she’s already got the measure of the Doctor and is eager to learn more about what he’s up to.

In the season’s opener, ‘The Pilot,’ we learn that the Doctor has been serving as a professor at a University in England, where Bill works in the kitchens and sneaks into his lectures when she is able. We later learn the Doctor has been there, with Nardole as his manservant of sorts, for 70 years. So what’s keeping him on campus, and why has a Time Lord who’s always known for travel, decided to stay put in one place for so long?

The answer lies in the depths of the campus, where a vault hides. In ‘The Pilot,’ it was revealed that whatever is in the vault is something that the Doctor and Nardole do not want revealed to staff or students, and something the Doctor is very keen to keep hidden from the entire universe. Of course, Bill made it her business to find the location of the vault, and now she (like us) wants to know what’s inside.

Peter Capaldi in Doctor Who season 10 Doctor Who Season 10: Whats in the Vault?

Whatever its contents, it seems to be related to a promise the Doctor has made. In the second episode of season 10, ‘Smile,’ the Doctor introduced Bill properly to the TARDIS and gave her the choice of whether she visits the past or the future. Just as she made her choice (the future), Nardole appeared, and chided the Doctor for the very idea of space and time travel. He gave a stark reminder that the Doctor is not supposed to “go off-world,” meaning that he’s meant to be on earth at all times. It’s unclear whether he’s been off-world before in the last 70 years, or whether Bill’s arrival has reinvigorated his desire for travel, but either way he’s supposed to stay put and guard the vault.

Why is Nardole so determined that the Doctor should fulfil his duties? It seems possible that Nardole is some kind of cyborg since he originally lost his head. Has he been rebuilt and sent to the Doctor from a higher being, such as River Song, for example? It could be that his whole purpose to make sure the Doctor carries his duties through to completion.

Although we don’t know what is behind the doors to the vault, we do know, thanks to Bill’s persistent questioning, that the Doctor is making good on a promise he made. Eventually deciding that he owes her an explanation, the Doctor explains, “A long time ago, a thing happened. As a result of the thing, I made a promise. As a result of the promise, I have to stay on Earth.”

This vault will be an ongoing mystery throughout the season and, as a result, we are unlikely to get any real answers until much nearer the end of season 10. Still, there are a few guesses we can make regarding what ‘the thing’ is that happened, and what the nature of the promise could be. The doors to the vault have a Gallifreyan symbol on them; hinting that the Doctor’s home planet is something to do with all of this. Either the contents inside are to do with the planet, or Gallifrey is responsible for the construction of the vault. Did the Doctor make a promise to his people that he would always guard whatever is inside?

For a long time, the Doctor believed Gallifrey was destroyed in the Great Time War, and he carried that burden, placing the blame upon himself. Though it was later realized that the planet had just been frozen and moved to another dimension, the Doctor’s commitment and care for the Gallifreans was clear. Now that the planet is back within the universe, it’s a safe bet that were anything, or anyone, to pose a threat to Gallifrey, the Doctor would be quick to intervene.

But what, or who, could be powerful enough to warrant being locked away in a vault on planet Earth, with the last remaining Time Lord standing guard? There are several possibilities, ranging from a weapon of mass destruction, to a violent and destructive monster.

One possibility, given what we know about the remainder of Doctor Who season 10, is that Mondasian Cybermen are present. They will appear in the last two episodes of season 10, and the Doctor’s regeneration will begin (at least) in the finale. Their appearance is at Capaldi’s request, since they are a favorite of his, so has Moffat engineered it so that they are the result of the Doctor’s demise? Obviously that vault isn’t holding an entire Mondasian army inside, but what if it serves as access to them, keeping them frozen in space and time, perhaps, and somehow they’re going to escape?

John Simm as The Master in Doctor Who Promo Shot Doctor Who Season 10: Whats in the Vault?

Then there’s the possibility that the vault holds another of the Doctor’s nemeses: The Master. John Simm will reprise his role, having last appeared in Doctor Who when David Tennant was serving as the tenth Doctor. Right now, Missy (Michelle Gomez) is the Master, and the two are supposedly unable to exist alongside each other. Tennant’s Doctor said he would keep The Master locked up, so perhaps Missy can only exist because the other Master is kept in a vault?

During ‘The Pilot,’ fans will have noticed two pictures on The Doctor’s desk – one of River Song and one of his granddaughter, Susan. It’s not yet known if Alex Kingston will be back during season 10, and it’s been kept well-hidden if she is, but there is also the very real possibility that whatever is in that vault, the Doctor is guarding as part of a promise he made to River, Susan, or both.

We can leave it to Bill to get to the bottom of it all, though. She’s a perfect reflection of the viewer in the number and type of questions she asks. The preview for episode three, ‘Thin Ice,’ showed her asking the Doctor if he’s ever killed anyone. It’ll be interesting to see whether he answers, or whether he tries to avoid the question. One other interesting point to note, is that a conversation between Bill and the Doctor that took place during ‘Smile,’ surely hints to everything going badly wrong concerning the Doctor’s abandonment of his post in the future:

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.

Yes, you just know the Doctor is going to get stuck in space traffic, and someone will find out.

Next: Doctor Who: Smile Review & Discussion

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