In its more-than-50-year shelf life, Doctor Who has always dug deeply into its past, exhuming iconic characters, monsters and situations - Daleks and Cybermen and Ice Warriors, oh my! - to the great delight of its fans.
In this year's Christmas-themed episode, Twice Upon A Time, the series stands on the precipice of a genuinely unwalked path - the casting of the show's first female Doctor Who - but also digs deepest into its canon, bringing back William Hartnell's first Doctor.
Hartnell died in 1975 so the role is played here by actor David Bradley; it is Bradley's second turn at Hartnell's Doctor, having played both actor and character in the Doctor Who "biopic" An Adventure in Space and Time, a drama about the origins of the series commissioned for its 50th anniversary.
Series producer Steven Moffat says the power of the first Doctor is that he comes from "before a time when he clearly identified his role in the universe. He's at a very different moment in his life. [And] there is a moment where he can be horrified by the almost monstrous Doctor that he's going to become."
Moffat points to a moment in the episode where the first Doctor and the present Doctor, played by Peter Capaldi, are working together. "The twelfth Doctor refers to the Earth being protected, and the first Doctor's going, what? Who by? He's already doing that job, but he doesn't know it yet," Moffat says.
The other guest star - an admittedly unexpected one - is Hartnell's Tardis. Within the Doctor Who canon, the Tardis is his time-travelling London police box. While modern Tardises have enjoyed lavish sets, the Hartnell-era Tardis is plain white, with circular roundels on its walls.
On that, says actor Peter Capaldi, there were as lot of excited people. "Who maybe don't even know why they're excited," he says. "There were a lot of smiling faces. Like they don't really know what's happened but there's something that echoes very deeply about those roundels and white console."
Moffat - describing himself as "a tragic fanboy" - adds that in such situations it becomes difficult to "tell if something is actually good or just pleasing me."
He notes that not a single designer he has worked with thought the roundels were good. "They all just go, no, that's terrible, that was the laziest decision you could make to put a bunch of circles on the walls. And I am going, don't say that because I loved them, absolutely loved them."
The Christmas episode, which is knitted in part to the Hartnell-era story The Tenth Planet, in which the Doctor has met for the first time the sinister, cloth-and-metal-covered Cybermen, brings the two Doctors together because they face a shared dilemma.
Regeneration - the process through which a Time Lord can replace an ageing or wounded body with a new one - is imminent. Each man is wrestling with the burden of change and an almost childish resistance to the idea of it.
"It's the torment of having to become somebody else in order to carry on," says Moffat. "You know the Doctor's a stubborn man. And he doesn't want to always turn into somebody else. Imagine you had to do that, it's irritating. You finally got the hang of this one, then look in the mirror and it's somebody else."
Capaldi believes his Doctor is wrestling with his apparent obligation to the greater task of keeping the universe safe.
"I think he's fed up with it," Capaldi says. "But he still has to carry on as a being. That being's job, as it were, is saving the universe. And I think that's a tough responsibility. The dilemma of the Doctor that I've been playing is that he would like to take his life into his own hands and stop.
"So he's got a decision to make about whether he should stop or be unselfish and continue for the benefit of the universe," Capaldi says.
What makes this particular gathering - Moffat, Capaldi and writer (and guest star) Mark Gatiss - so unusual is that all are, in various forms, proper fanboys. Each has a childhood tightly knitted to Doctor Who and each has, in different ways, been given the keys to the kingdom.
To play in the toybox of one's childhood is a joy says Moffat. And though it seems surprising, he adds there is no innocence lost.
"I can see why you would think that would be the case, but it sort of isn't," Moffat says. "Doctor Who fundamentally is so simple. It's the Doctor and the police box and he can go anywhere in the universe. And it's bigger on the inside, and he fights monsters. That's it. That's the whole show. So we've done our version, which had particular inflections and ideas in it."
With Moffat and Capaldi bowing out, Moffat says, it "just reboots back to the Doctor, the box, and the adventures and the monsters. So that's a different show again. A different show that is in most significant ways, utterly identical to the way it has always been."
Moffat says he will "quite quickly just be back behind the sofa just watching it. I'm used to the idea of Doctor Who being a show made by other people. And it will be, while importantly identical, also utterly different."
For Capaldi, the little boy who once wrote fan letters to his favourite Doctor Who, walks away with joy in his heart.
"It takes a lot of work, and you can't play the part by just being a fan," Capaldi says. "It's not how it works. It's not how I work. I couldn't do it that way. I had to just try and approach as an actor of my experience and try and give it everything I could. So I think I will probably reflect on what it was like to the child later.
"At the same time, it's just delightful to have been involved with it, and to want any more, it's being greedy. I'm just happy to have been there. People may make that what they may."
WHAT Doctor Who Christmas Special
WHEN ABC, Tuesday, 7.30pm