Doctor Who fans are convinced they know who is inside the vault

This series of Doctor Who has centred around a mystery vault the Doctor (Peter Capaldi) and Matt Lucas' Nardole are guarding – but what (or, more specifically, who) is inside it?

That's the question bugging Whovians as we continue through series 10 – but last night's episode offered a crucial clue as to what lurks behind that intricate door.

In the closing moments, we saw the Doctor bring a bag of Mexican food for dinner with his guest/prisoner. As he took over the watch from Nardole, we heard the tinkering of a piano – a melancholy rendition of Fur Elise.

But when the Doctor promised to tell a tale about children being eaten, the tune switched to a bouncy version of Pop Goes the Weasel. Whoever's inside clearly likes bloodthirsty stories.

For Twitter, it all added up to a fan favourite character who we already know is returning in series 10: yes, Missy.

Michelle Gomez was glimpsed in the trailer released at the start of the series and viewers have put two and two together and decided she's the one playing the piano...

Yup, seems about right.

Everyone's pretty convinced.

Although, the only thing as good as Missy would be...

Doctor Who continues next Saturday at 7:15pm on BBC1