Transmission: 28.11.2015.

Writer: Steven Moffat.

Director: Rachel Talalay.

Guest Starring: Jami Reid-Quarrell.

The Doctor is trapped and alone.

Stuck in a seemingly-endless castle, nightmarish remnants of his past surrounding him and a photo of his dead companion staring down at him, this is a challenge the like of which he's never faced before. Teleported from the Trap Street, he finds himself pursued by the deadly Veil.

And if he survives, his reward will be the one place he's longed for.

"I confess," he says. "I am afraid."

This is the penultimate episode of Doctor Who Series 9, and sees the mysteries of the previous 10 episodes unfold. What is the Hybrid that's been talked about since the Doctor faced Davros on Skaro? The Doctor knows... Is Clara really gone? And what are the contents of the Time Lord's Confession Dial?

This is the very first 'one-hander' in Doctor Who history: that is, aside from the sole monster, only Peter Capaldi's Twelfth Doctor features. "It's the first time we've ever done an episode like it," Brian Minchin, executive producer, says. "And it's the most virtuoso, fantastic piece of writing from Steven [Moffat, showrunner]."

"It occurred to me that I had just written a huge monologue for him at the end of [The Zygon Inversion]. Ten minutes. I was typing forever in that," Moffat explained to Radio Times. "And I just thought, 'maybe we don't need anyone else – we don't need to cut to anyone nodding – it's just him?'"

Capaldi adds that it's "undiluted Doctor Who" that reminded him of The Mind Robber, the 1968's Second Doctor classic. Indeed, it'll be as experimental as that story, as well as recent outings like Midnight, Listen, and Sleep No More.

This concluding two-parter is the directorial return of Rachel Talalay who helmed last year's finale, Dark Water/ Death in Heaven, and she enthused to Doctor Who Magazine: "I was relived and delighted when the invitation came in, to come back. I had no idea how different it was going to be this time in terms of scripts. They couldn't be more different o last year, or to each other – a two-parter where each part is so completely different."

It's also the return of Jami Reid-Quarrell, the only other actor apart from Capaldi in Heaven Sent: we last saw Jami as Colony Sarff in the Series 9 opener, The Magician's Apprentice/ The Witch's Familiar.

So will the Doctor survive this never-ending maze? What is the Veil? How will Series 9 be wrapped up?

Heaven Sent airs tonight on BBCOne at 8:05pm, with a duration of 55 minutes.