The new issue of Doctor Who Magazine has confirmed that fans won’t have to wait too long to clear up the mystery of what’s inside the vault.

It will be revealed in Extremis, episode 6 of Series 10, which will premiere 20 May.

Steven Moffat explained: “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.”