What’s inside the Vault the Doctor has been guarding so far in Doctor Who Series 10? While many of us thought it’d be this season’s arc, that we won’t find out until the finale begins, Steven Moffat has revealed that we’ll actually discover the truth very soon.
On 20th May, in fact!
That’s when Extremis airs. Yep, the doors will be opened in Episode 6 of Series 10, as the showrunner told Doctor Who Magazine:

“It’s the absolute classic mystery, isn’t it? It’s the locked door. It’s the simplest thing I could do – the Doctor’s got a locked door – but it’s instantly fascinating. I mean, you’d only have to be staying in someone’s house for a while and realise that one of the doors is locked to become impossibly curious about what’s in there. That’s human nature. There is something perfectly fascinating about a locked door.

“And we don’t have to wait a whole 12 episodes to find out what’s in there, which I think is important, or you’d go mad.”

Phew. It is a relief that the vault will be open so soon, especially considering it’s been teased since The Pilot, and the most recent episode, Thin Ice had a considerable coda, as Nardole (Matt Lucas) was scared by something knocking on the other side. Knock Knock, too, includes a clue.
Some reckon it’s John Simm’s Master (though he’s not due in the series until World Enough and Time/ The Doctor Falls), or perhaps Missy, played by Michelle Gomez. Maybe something to do with the Truth Monks?
But the DWC has it figured out. Oh yes. And it’s really obvious what’s in that vault.
So what do you reckon? What’s that come a-knocking? And are you pleased we find out so soon?