Doctor Who Series 10: 7 Big Questions We're Asking After 'Smile

4. What Was The Event Lying Behind The Promise?

While he takes a ‘quick’ excursion in the TARDIS with Bill, the Doctor leaves Nardole guarding the mysterious vault. And that’s the last we see of Matt Lucas’s character in this one, meaning that very little is given away about what the vault might contain.

The only extra clue we get here is that ‘a thing’ happened causing the Doctor to make his promise to stay on Earth and watch over the vault. The most we can assume is that the promise was made to somebody other than himself and that given the Doctor’s wayfaring nature, the event must have been pretty cataclysmic. Whatever is in that vault is tied to something that happened in the Doctor’s past.

It could of course be an off-screen adventure like his encounter with the love struck Algae Emperor (now that’s surely a story probably best left to the imagination), but given the event’s undoubted significance it could well be something we’ve seen before.

The Time War and specifically the events in The Day of the Doctor might be worth a bet. After all, Moffat teased us with the first ever shot of the Twelfth Doctor back in the 50th anniversary epic, when in tandem with his other selves he helped to save Gallifrey by moving it into the pocket universe. Could the vault be hiding Gallifrey itself?

It’s unlikely, given that the Doctor found Gallifrey (Hell Bent), losing his memory of Clara in the process. Tragically, if he ever remembered her again he’d stop being the Doctor. Which brings us to another tantalising clue from The Day of the Doctor. The Doctor tells Clara his name was a promise. Perhaps the vault contains his memories of the impossible girl. What better reason could Missy have for opening it again?