This week's episode of Doctor Who, Heaven Sent, is one of the boldest stories the show has ever attempted: a single-hander carried entirely by one actor – Peter Capaldi as the Doctor. 

"It's a mad thing to do," says showrunner Steven Moffat. "I'm yet to find out whether I've just gone mad or not. It occurred to me that I had just written a huge monologue for him at the end of [episode eight, The Zygon Inversion]. Ten minutes. I was typing forever in that. And I just thought, 'maybe we don't need anyone else – we don't need to cut to anyone nodding – it's just him?'

"It just puts it in a different place. If you're not going to try things that scare you a bit, that scare everybody a bit, then it'll feel what it must never feel – it'll feel like an old show. I remember when Russell T Davies first brought it back, he said, 'we can't ever make it feel old. It always must feel like a brand new show.' And that worked, and we carried on. But we've now got to the point where the new show is also old. Slightly marred by the 50th anniversary being two years ago, but the new show is ten years old. So you're fighting that as well. You want to be doing things that are first/second series experimentation."

The episode follows last week's Face The Raven, which saw the death of companion Clara Oswald and the betrayal of the Doctor, who – according to the official synopsis – has been teleported to, "a world unlike any other he has seen." It's here that he faces, "one final test. And he must face it alone. Pursued by the fearsome creature known only as the Veil, he must attempt the impossible."

It's an unusual episode, even for such a wide-ranging show, but according to Capaldi the story is about as Doctor Who as it gets.

"I was excited because it hadn’t been done before but it seemed like a very Doctor Who idea," he tells us. "It in one way reminded me of [classic story] The Mind Robber. It’s a kind of very internalised adventure. I love things that seem to be 100% undiluted Doctor Who. And that’s what this episode seems to be to me. It is challenging because you realise that if the audience get bored with me that’s really bad news. But there are more colours and angles to it than I’m suggesting..."

Heaven Sent will air on Saturday 28th November at 8:05pm on BBC1