Extremis, the sixth episode of Doctor Who series ten, has just aired on BBC1. Radio Times reviewer Patrick Mulkern calls it "ingenious, breath-taking television" and has asked its writer, the showrunner Steven Moffat, to talk about the episode in detail. SPOILERS follow...

RT’s Patrick Mulkern: Cardinal Moffat, apologies if it seems I’m blowing incense up your cassock, but Extremis is a remarkable piece of television. Unusual storytelling in that it is largely two interwoven stories: one set “A Long Time Ago” dealing with Missy’s execution; the other happening in an alien simulation of the now.

You told me that you were worried by the “sheer head-f***ery” of this one. It’s mind-bending in a rewarding way – because everything should fall into place for viewers by the end. Extremis answers some mysteries and sets up more for the rest of the series. Perhaps you could expand on what you wanted to achieve with this episode…

Steven Moffat: Oh, I don’t know. Once a year – sometimes more than once – I try to push things a little bit. I started this season with a very solidly traditional approach, and a huge focus on just the Doctor and Bill.

Which is harder than you think.

The story of the week tends to overwhelm in this show, and that can be a killer when you’re trying to spotlight a new character. So the big note for me, and Frank [Cottrell-Boyce] and Sarah [Dollard], as we wrote our scripts, was to shove everything aside and make it about the two of them, come what may.

Frank opted to make his a two-hander for much of his episode’s run – which I worried about when he suggested it, but later adored – and Sarah took the bold and brilliant decision to not even explain what the monster was! That seems to break every rule, but did you even notice? Two advantages: one, we are spared lines like “It’s a Fanganoid for the planet Bing” and two, we learn a huge thing about the Doctor and Bill: for both of them, it doesn’t matter what it is; it matters only that it’s in chains. Loved that – very Sarah!

Also, I’ve always wondered why people keep travelling with the Doctor, when they encounter so much death and fear. He promises them wonders, and gives them tunnels and screaming death! Why don’t they go straight home? If your first experience of the wider universe was a Jamie Mathieson horror fest, you’d probably never come out your bedroom again.

So I wanted Bill, for the first few weeks, to encounter the gentlest, kindest sort of Doctor Who. The Doctor is the wise old man of the universe, fixing things when they go wrong – throwing back the curtains, letting in the sunshine, showing everyone that the universe isn’t evil, you just need to understand it better.

So, by the time we get to Oxygen, Bill is properly bonded with the Doctor and his lifestyle, and is ready to learn more: the Doctor can be a reckless thrill-seeker and those are real monsters slouching towards you! If that had been her first experience, she’d have left after episode one. I so wanted Bill to be one of us – which meant we had to give her a reason to stick around with the madman in the box. 

Sorry, I’m rambling. But the point is, by the time we get to episode six, I thought it might be time to go darker and stranger. Time to get odd, even experimental. Now and then, we mess about with the format: Heaven Sent, Listen – Russell [T Davies]’s entirely brilliant Midnight is probably the best example. From the classic series there’s The Mind Robber, Warriors’ Gate, Kinda, Ghost Light.

If you don’t like any of those shows (and fair enough, some people don’t) I think you’d probably agree that Doctor Who is richer for attempting them. Now, I’m worried I’ve drawn some very big comparisons there, and as I type, Extremis hasn’t even aired – maybe everyone will hate it. Certainly some people will, and I’m properly, sensibly nervous. But it’s an attempt at pushing the envelope. Bending the show, trying not to break it. Crossing my fingers. While typing. Explains a lot, really, doesn’t it?

There are theories – taken semi-seriously by clever people – that what we are living in is a simulation (which would certainly explain why my coffee cup is never where I left it). Now for me, the most interesting thing about that is the idea that a simulation could be smart enough to figure out that it’s a simulation. Weird, kind of tragic.

Imagine feeling real, and discovering that you aren’t? I did that with the kids in Forest of the Dead. But if you put clever old Doctor Who inside a simulation, I bet he’d not only figure it out, he’d fight back, and send a message to the real world, warning them they’re being simulated. 

That was the idea. Hope you enjoyed. If you didn’t, there’s a space pyramid on the way. And Ice Warriors. And Mondasian Cybermen, and more Missy, and John Simm’s Master. Damn it, we’ve even got Aberdeen. 

Mulkern: You’ve shown how the Doctor and Nardole were reunited and took charge of the mysterious vault. Why did you decide to reveal, at the midpoint of the series, that it is Missy (Michelle Gomez) in the vault? I noticed you don’t actually show her inside it – is there a twist to come?

Moffat: Well, it’s definitely Missy in there, we’re not fooling you about that. The story now is what’s going on between them. That dangerous friendship, burning away  – who else will be lost to the flames? “Show me how to be good,” she asks him. Will the number one liberal do-gooder of the universe be able to resist?

And we gave it away early because I know Doctor Who fans – they’ll have had it figured out by the end of episode four!

Mulkern: I’m hungry for some more Missy now – and that's not her done with until the finale... 

Moffat: Plenty more Missy to come. Hopefully, when you least expect her. But she’s back in play so trust nothing and no one.