In the original script for The Tenth Planet the Doctor refused to give in to his regeneration but it was not cut as director Derek Martinus was running short on time.
Mark Gatiss plays The Captain, his second credited role on Doctor Who- he played Lazarus in The Lazarus Experiment though he has had several uncredited appearances too- Gantok in The Wedding of River Song and the voice of Danny Boy in Victory of the Daleks and A Good Man Goes to War. He has also written a number of episodes (The Unquiet Dead, The Idiot's Lantern, Victory of the Daleks, Night Terrors, Cold War, The Crimson Horror, Robot of Sherwood, Sleep No More and Empress of Mars). He was the only Doctor Who writer to have appeared in the show until this episode.
Gatiss is joined here by fellow writer Toby Whithouse (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) who plays the German soldier.
The Captain's name is revealed to be Captain Archibald Hamish Lethbridge-Stewart, an ancestor of long-running Doctor Who character Brigadier Lethbridge-Stewart and his daughter Kate Stewart.
The First Doctor complains that he thought his future self would be younger, a play on the fact both Peter Capaldi and William Hartnell were fifty-five years old when cast as the Doctor. Hartnell was 55 years, 10 months and 15 days old on his first appearance whilst Capaldi was slightly younger at 55 years, 7 months and 9 days old. Capaldi stayed in the role longer than Hartnell though and is nearly a year older than Hartnell was when he regenerated. David Bradley is much older than both at 75 years old.
The role is David Bradley's second in Doctor Who having played Solomon in Dinosaurs on a Spaceship. He played William Hartnell in the 2013 docu-drama An Adventure in Space and Time about the early days of Doctor Who. It was his role in this that led to Peter Capaldi's suggestion that he appear on the main show as the First Doctor.
Rusty the dalek returns here after his appearance in the Twelfth Doctor's second story Into the Dalek. His first line in Twice Upon A Time is the same as his last line of his first appearance: "I am not a good dalek. You are a good dalek".
This is not the first time the Doctor has been to the planet Villengard. Back in The Doctor Dances the Doctor noticed Captain Jack's weapon blaster came from there and said he destroyed the weapons factories and a banana grove replaced them.
Professor Helen Clay is from New Earth in the year 5,000,000,012 which is eleven years before the episode New Earth and 41 years before Gridlock.
The Twelfth Doctor has several nicknames for the First Doctor. Mary Berry refers to the former star of The Great British Bake Off who now makes various programmes for the BBC and Corporal Jone refers to Clive Dunn's character in Dad's Army. Mr Pastry was an accident-prone comedy character played by Richard Hearne, who was considered to play the Fourth Doctor.
On the First Doctor's console the name 'Bernard Wilkie' is fixed on tape. Wilkie was a visual effects designer in the early days of Doctor Who and also worked of other BBC series like Quatermass and the Pit.
We see a montage of clips from Doctor Who's past which includes footage from Invasion of the Dinosaurs, Arc of Infinity, The Happiness Patrol, The Parting of the Ways, Journey's End, The Waters of Mars, Night of the Doctor and The Day of the Doctor.
We also hear a number of the Doctor's nicknames. The Oncoming Storm (first used in The Parting of the Ways, the Imp of the Pandorica (referring to The Pandorica Opens), the Shadow of the Valeyard (the villain of The Trial of a Time Lord was a version of the Doctor), the Beast of Trenzalore (referring to The Time of the Doctor), the Butcher of Skull Moon (based on a Time War event mentioned in Hell Bent), the destroyer of Skaro (the Seventh Doctor helped destroy Skaro in Remembrance of the Daleks, though the planet has been seen several times in the new series) and the Last Tree of Garsenon, which we've not heard before.
In his final speech the Twelfth Doctor also uses the phrase "never be cruel or cowardly". This was first used by Doctor Who writer Terrance Dicks in his book The Making of Doctor Who: "He never gives in, and never gives up, however overwhelming the odds against him. The Doctor believes in good and fights evil. Though often caught up in violent situations, he is man of peace. He is never cruel or cowardly".
He also says "never eat pears" which is a reference to a line cut from Human Nature (Martha fast-forwards through it) which was originally used in Paul Cornell's novel of the same name. He also told Clara in Hell Bent not to eat pears as "they're too squishy and always make your chin wet."
The Doctor talks about children being able to hear his name and this is something that comes from Peter Capaldi himself. In April 2017 a young fan asked what the Doctor's real name is this is Capaldi's reply.
"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 your heart's in the right place, you'll hear it"
The episode ends with Jodie Whittaker's Thirteenth Doctor falling from the TARDIS, the interior of which is exploding. She should survive the fall as the Tenth Doctor showed in The Christmas Invasion that Time Lords can still heal for a while after their regeneration. The TARDIS is likely exploding so that it can be given a revamp for the new series.
