"Twice Upon a Time" is a story about memory and taking stock, about working out who you are and what your purpose is, and a story about sometimes being blind to your own beneficial impact on the people around you.

It's also Peter Capaldi's swansong from Doctor Who, featuring a welcome return from the First Doctor, Mark Gatiss as a close family member of one of the Doctor's oldest friends and the first time we get to see the Thirteenth Doctor in action.

Here are a few things that you should keep an eye out for while you watch:

The section of the First Doctor adventure "The Tenth Planet" which opens the story is taken from the only surviving clip of that particular episode. It's taken from part four, which is one of 97 episodes of classic Doctor Who that were lost in space saving measures by BBC archivists. Quite by chance, the clip was used in an episode of the children's magazine show Blue Peter, which has survived, and that's how we still have footage of the first regeneration in Doctor Who history.

The First Doctor did indeed once utter the line "What you need is a jolly good smacked bottom!" during "The Dalek Invasion of Earth," but it was to his granddaughter Susan. He's not on record as offering similar capital punishment to any of his traveling companions.

https://www.youtube.com/watch?v=snprb1JY_YA

The Captain faces a German soldier in the bomb crater, who is played by Doctor Who writer Toby Whithouse. His writing credits include "School Reunion," "The Vampires of Venice," "The God Complex," "A Town Called Mercy," "Under the Lake" / "Before the Flood" and "The Lie of the Land", but he also acted in Bridget Jones’ Diary.

Torchwood fans may well recognise Nikki Amuka-Bird, who plays Helen Clay, the glass woman from Testimony, as she also appeared as Beth Halloran in "Sleeper," the second episode of the second series.

The First Doctor's TARDIS has the name Bernard Wilkie affixed above a bank of switches, this is a belated nod for a visionary BBC designer whose contribution to the first episode—"An Unearthly Child"—went uncredited.

https://twitter.com/StuartHumphryes/status/942532581140156417

In the original script for "The Tenth Planet," the First Doctor was indeed supposed to refuse to accept his regeneration, with the line "No, I can't go through with it! I can't. I will not give in!" This didn't make it to the screen for reasons of time, so the idea of him avoiding regeneration got somewhat lost, until now.

The Doctor mentions that his name can only be heard by children, under the right circumstances, which follows a beautiful answer Peter Capaldi once gave to a young fan asking what his name may be: "I don’t think human beings could even really say his name. But I think we might be able to hear it. At a certain frequency. If the stars are in the right place, and you heart’s in the right place, you’ll hear it."

The Twelfth Doctor calls the First Doctor Mr Pastry, a fairly arcane reference by modern standards, to the clumsy character created by music hall performer Richard Hearne. Hearne has some history with Doctor Who, in that he was under consideration for the role of the Fourth Doctor in 1974. Producer Barry Letts chose Tom Baker instead, after it became clear that Richard intended to play the part using Mr Pastry's comic chops.

https://www.youtube.com/watch?v=36N9CUhIG7s

Just before he's taken away by the Testimony, the Captain cites the maxim "war is hell," which is credited to William Tecumseh Sherman, Union Army general. On April 11, 1880, in front of a huge crowd in Columbus, Ohio, he is reported as having said: "There is many a boy here today who looks on war as all glory, but, boys, it is all hell."

The Twelfth Doctor also refers to his earliest incarnation as Mary Berry, who was, until recently, one of the judges on The Great British Bake Off. She has published more than 75 cookery books—2009's Baking Bible was a bestseller—starting with The Hamlyn All Colour Cookbook in 1970. It must be the hair he was thinking of:

https://www.youtube.com/watch?v=bDxYm4-eHuA

Oh and an extra one, because it's Christmas: This is not the first multi-Doctor story to contain the words "oh" and "brilliant" used to to denote excitement. In "Time Crash," the Comic Relief short in which the Tenth Doctor meets the Fifth, Ten celebrates the moment by saying: "Oh brilliant! I mean totally wrong. Big emergency. Universe goes bang in five minutes but, brilliant!"

Now go back and read the entire 10 Things You May Not Know About Doctor Who archive.