10 Doctor Who Mysteries With The Worst Possible Answers

6. What's Inside The Vault?

Did Series 10 really need the vault? The Twelfth Doctor teaching at a university was a great premise that could have ran for years, so there was no need to give him an excuse to be Earthbound.

Still, the vault and its contents provided the narrative backbone for Series 10, and it provided a decent enough mystery for fans to obsess over for the first half of the series. Is it Missy? Surely not, too obvious! Ah, but what if it's John Simm's Master – we already know he's coming back thanks to the BBC needlessly ruining it! Or maybe it's the Doctor himself? After all, the Doctor usually ends up locked in a big box in Steven Moffat's Doctor Who.

Extremis confirmed what we'd all figured out from the off – that it was Missy, locked inside after the Doctor swore to keep an eye on her. So obvious was this revelation that Extremis doesn't even try to build up to it, instead giving us the answer in literally the opening moments.

Now of course, it was a delight to have Michelle Gomez be part of the series, so that in itself was a big plus. It's just that, given that Steven Moffat is a writer who never takes the obvious path, it was really disappointing that he went so route one with this mystery.