"A lot of heart, but a lot of brain too" - could the Doctor Who Christmas special feature a return for a fan favourite?

"We've just started working on the Christmas episode,” Moffat told BBC Newsbeat.

"It's got a lot of heart, but it's got a lot of brain too. Figure that one out.”

Ok Mr Moffat, challenge accepted...

Now, we think we’ve got the heart reference down – the Doctor, as with all Time Lords, has two hearts, which could reasonably be described as “a lot”. So we reckon he's definitely in it... But what could Moffat mean about the episode having a “lot of brain?”

Was it just an offhand reference to the fact that it's an adventure with a particularly fiendish plot as well as plenty of warmth, or a nod to something more specific that happens in the episode?

Matt Lucas's Nardole as one of the heads imprisoned by King Hydroflax's body

We think the latter – and we might have an idea what he’s referring to. In last year’s Christmas episode the primary antagonist was the robot body of King Hydroflax (voiced by Nonso Anozie), who had a habit of severing people’s heads and containing them within his chest cavity, switching them out for use when necessary.

One of his victims was Matt Lucas’ Nardole, who has since been confirmed as returning for series 10 in full possession of his own torso. Could this be how the character is reintroduced to the action, with his and many other “brains” still trapped inside the robot body and needing to be rescued by the Doctor?

You have to admit, it sort of makes sense – they’ll need to explain how Nardole gets his body back after the conclusion of The Husbands of River Song left him happily muddling along with his new condition – but it could be that we’re still barking up the wrong tree.

Could it instead be that the “hearts” and “brains” comment hints at the return of the Doctor’s fellow brainy (and two-hearted) Time Lord Missy (Michelle Gomez, above)?

Or perhaps a surprise comeback for revived series alien the Ood and their three-brained system of thought? No?

What we do know from other comments Moffat made is that the production team have “four episodes in the can” already, and that new companion Pearl Mackie is "a breath of fresh air on the Tardis". So even if the plot is still shrouded in darkness, the future of Doctor Who is sounding pretty bright.

Doctor Who will return to BBC1 this Christmas before a full series in Spring 2017