‘Either we change and go on or we die as we are.’ Moffat’s second go at a farewell story is a different beast than The Husbands of River Song. That was about finding acceptance in songs ending, after the Doctor had fought so hard to prevent Clara’s and nearly lost himself in the process. This is about letting go, giving someone else the chance to sing, even when it would be easier to call it a day.

In a sense that’s perfectly natural: The Husbands of River Song was just meant to be Moffat’s last episode, not Capaldi’s (and means Capaldi’s best run takes place between Moffat’s two last stories for him). But this must serve as a double exit, which makes it initially quite perplexing when Moffat introduces the first Doctor into the mix. He does it very cleverly, having both Doctors arrive at their endgame after a fatal encounter with Mondassian Cybermen (sketched in here via an efficient mix of clips and re-enactments of The Tenth Planet), but inevitably the presence of two Doctors means Capaldi has to share some limelight.

Except, this is David Bradley rather than, say, Tom Baker or Paul McGann. David Bradley is a fine actor, but he’s only previously played the first Doctor as William Hartnell in character in An Adventure in Space and Time, so he’s not been explicitly brought back to do his party pieces. Instead, he’s there to bring to life the idea of “change or die”, confronting the 12^th Doctor with the reality of how much he’s already changed, how far he’s come, since the early days.

This wouldn’t work nearly as well with later Doctors who are already on the journey. It has to the first, unreconstructed version. Which also means Bradley gets saddled with a lot of unsubtle (but funny) dialogue to land this point with Christmas Day 2017 audiences unfamiliar with the critique of sexism in the classic series. In the end, ‘I will not change’ is the rejection of hope that you might become something better. And by meeting each other at this point, both Doctors realise that it’s a no-brainer, that death is a permanent solution to temporary fear or weariness.

This is explored mostly through three long dialogue-heavy sequences in the TARDIS, the Chamber of the Dead and a planet in the distant future. Luckily, it’s some of Moffat’s best dialogue. Once the unreconstructed humour is out of the way, Bradley’s first Doctor is much closer to the spirit of scientific enquiry in Hartnell’s Doctor: poking about discovering the truth of the untinned Daleks and searching to understand what makes good triumphs when in theory evil should. When asked to explain why he fled the Time Lords, he handwaves it, ‘There were many pressing reasons’. That sounds exactly like the kind of thing the first Doctor would say. I’m also a huge fan of Archie’s words to Bill, the nearest thing we get for an explanation of the 12th Doctor’s death wish: ‘One doesn’t want to die, of course, but one gets in a certain frame of mind… But… I’ve lost the idea of it.’ There’s “timey wimey” as a joke with the brandy bottle, and a different spin on the meaning of ‘Doctor of War.’

Then there’s the last sequence of the Doctor, Bill, Clara and Nardole on a battlefield that looks like the one he’s just left on the Mondas spaceship. It has a poignant beauty in the dialogue, ‘A life this long, do you understand what it is? It’s a battlefield, like this one, and it’s empty’, and honesty in the performances; no dramatic grand shots or weepy close ups. It plays out like a true end of the road for these characters. Had this been followed by a brief regeneration sequence, perhaps ending on, ‘I suppose one more lifetime won’t kill anyone. Well, except me’ I think it would have been perfect, especially accompanied by a slight return for Murray Gold’s Chancellor Flavia theme.

Sadly, we then get an extended spin round the TARDIS and a lot more dialogue about only dogs and children being able to hear the name of the Doctor, which adds nothing but time. But those last words, ‘Doctor, I let you go’, are perfect. You could hear them as a last retort to RTD/Tennant’s ‘I don’t wanna go’, and I suppose they are. But they’re also the right end to this story, acceptance that letting go and moving on is always better than holding tightly to the past.

This is the best regeneration episode since The Parting of the Ways. Unlike The Time of the Doctor, Moffat left no lingering plots to wrap up, and the threat here isn’t a grandiose, world-shattering evil plan – just Professor Clay’s final act of kindness, which more or less sums up Twice Upon a Time. Yes, there are some common Moffat flaws you can point out (it’s a very static, talky story; the threat is a “broken spring” rather than a villain). But on balance, it’s a very truthful last word from one of the great contemporary TV writers. The show has been lucky to have two of them in a row. Chris Chibnall, like Jodie Whittaker, has got big boots to fill.

Next Time: The Woman Who Fell to Earth