Warning: Spoilers for Doctor Who are in play. If you haven't watched "Extremis," catch up before you read on.
The moment has finally arrived, Whovians! Doctor Who has given us the answer to who's in the vault, and it's none other than Mistress Missy herself. While it was kind of predictable that this result would be the one that Steven Moffat would put into play, there's a lot of interesting details that factor into just why The Doctor has been put in charge of guarding Missy, and why exactly she's in the vault.
It's revealed at the beginning of this week's episode, "Extremis," that Missy was set to be executed for crimes that she had committed, courtesy of a machine that not only stops both hearts of a Time Lord, but also suspends regeneration. Of course, leave it to the now late River Song to stop The Doctor from executing his nemesis, as she sent Nardole to prevent him from carrying out his orders. Naturally, The Doctor shirked his responsibilities as an executioner, and made it so that Missy lived, leaving her in the vault to be guarded for 1,000 years, per his pledge.
While the reveal was certainly predictable, it does set up the recently announced departure of series star Michelle Gomez. With Missy possibly mortal, and without regenerative properties, she might be stuck with this form -- only to die and totally be wiped off the face of Doctor Who. If tonight's revelations have set us up for the truth, then Missy will more than likely fulfill her promise and reform herself, dying a hero's death as she did previously when she looked like John Simm.
But wait... John Simm is supposed to be coming back as The Master season, so how the hell does this leave him a path to return? One of two things could happen, thanks to the execution ritual, and the vault's inclusion: either Simm's Master is going to be brought back through some sort of time travel chicanery, which would allow the character to go on, or he'll be brought back in a flashback. If it's a flashback, it would be fitting as it would show the last time The Master sacrificed himself for The Doctor's well-being.
Of course, this week's big cliffhanger left us with The Doctor telling Missy exactly what he shouldn't have, letting her know he's blind and that he's going to need her help. The vault is still not open, and there is something much more frightening than Missy to contend with. As "Extremis" had revealed, there's a parallel universe that has been operating as a testing ground for an alien race that's looking to invade our world. So naturally, with the mystery of the vault solved, the new question of the hour seems to be, what's the new coming darkness that The Doctor seems so mortified about at the end of this week's episode?
Well, it looks like another week of thinking is ahead of us, as Doctor Who has left us hanging until next Saturday's episode. Tune in at 9 PM EST on BBC America, as the next turn in the Twelfth Doctor's final season reveals itself.