Twice Upon a Time (TV story)

Twice Upon a Time was the 2017 Doctor Who Christmas special. It was the final episode under showrunner Steven Moffat's tenure, and featured the last regular appearance of Peter Capaldi as the Twelfth Doctor.

Concluding the end of the Twelfth Doctor's life teased in World Enough and Time and shown throughout the events of The Doctor Falls and this episode, the ending showed the Doctor's regeneration and introduced Jodie Whittaker as the Thirteenth Doctor, marking the first female incarnation of the Doctor in the series history.

After a surprise appearance at the end of the last episode the First Doctor had his first full appearance on the programme since The Five Doctors. Here, he was portrayed by David Bradley, who had previously played the original actor William Hartnell in An Adventure in Space and Time. This was the first televised interaction between a "classic" Doctor and a BBC Wales era Doctor since 2007's Time Crash. Archival footage from The Tenth Planet is used to bookmark the First Doctor's place in this story. The special also introduced Archibald Hamish Lethbridge-Stewart, credited as "the Captain", a new member of the Lethbridge-Stewart family.

Ben Jackson and Polly Wright make a brief re-reappearance in the programme for the first time since their exit in 1967's The Faceless Ones. Bill Potts joins the two Doctors, and Captain Lethbridge-Stewart, as a Testimony glass avatar. Past companions Nardole and Clara, briefly, make similar appearances, the latter of which the Doctor regained memories of after having lost them previously in Hell Bent. The "good Dalek" Rusty, from Capaldi's second episode, Into the Dalek, makes his second appearance—in Villengard, a location mentioned only once before on television, in 2005's The Doctor Dances.

Synopsis
In the Twelfth Doctor's final chapter, he must face his past to decide his future, discovering hope in his darkest moment. But can he convince his original self to also change?

Plot
The episode opens as a flashback to when the First Doctor combated the Cybermen from Mondas. He is freed from his restraints by his companions Ben and Polly, the threat over. However, he is weak, his body beginning to regenerate. Refusing to give in, he leaves his friends to be alone. He wanders the South Pole, mumbling to himself that he will not change. Nearing his TARDIS, he hears a voice in the distance declaring themselves the Doctor. He approaches the man, who is knelt down in the snow, outside his TARDIS, rebuking his statement as he declares he himself is the Doctor.

Things shift back to the present, from the Twelfth Doctor's perspective, who gets to his feet, shocked at seeing the First Doctor. The Doctor is delighted at meeting his first self, but realises that he is refusing to regenerate. The First Doctor is confused and dismisses his future self as another Time Lord, before noticing the TARDIS. The Doctor reacts joyously at the First Doctor calling the TARDIS the ship but he dismisses him, too busy puzzling as to why it is suddenly bigger with larger windows. The Twelfth Doctor retorts him, stating it happens as the TARDIS can't "keep [her] tummy tucked in" forever. However, the snow suddenly ceases to fall, capturing the pair's attention. In the distance, another man in World War I attire addresses them, asking if one of them is a Doctor. The Twelfth Doctor looks at the man and asks "are you trying to be funny?"

The scene then shifts to the soldier's perspective, flashing back to 1914, Ypres on Christmas day. The soldier, a captain in the British Expeditionary Force is in a shell hole in the battle field, his revolver aimed at a German soldier, who also has his sidearm trained on the Captain. The Captain speaks English to the man, wishing they spoke English too or he spoke German. Both men realise they will kill each other in self-defense, their respective troops far away in their trenches, unable to assist either of them. Just as the Captain prepares to accept his fate and fire, a wave of energy flows through the field, freezing everything except the Captain. The Captain exits the shell hole, wandering around in wonder at the instance. A bright light suddenly flashes, capturing the Captain's attention. It suddenly encapsulates him, the soldier finding him in a room as lights flash with only "timeline malfunction" filling his eyes. He suddenly collapses to the ground in the arctic, hearing the two Doctors discussing the static snow. He approaches them.

The scene once more shifts back to the Twelfth Doctor's perspective. A portal of light opens in the distance, the two Doctors approaching it. The First Doctor orders the portal to reveal itself, while the Twelfth Doctor claims the planet is protected, surprising the First Doctor as he does not know who guards it. However, the light quickly vanishes, surprising the Twelfth Doctor as "that almost never works." The First doctor returns to the Captain, telling him to enter his TARDIS, the Twelfth Doctor coaxing him to follow. They enter, the First Doctor becoming surprised at seeing the desktop theme so different; he fears he has been burgled. The Captain is too in shock, but more in surprise at how much bigger it is on the inside. The Twelfth Doctor attempts to convince his younger self they are the same person, even displaying he too is regenerating. He even points out that the First Doctor's TARDIS is 70 feet away, proving the one they are in is actually a much older version, hence the size increase. Regardless, the First Doctor remains skeptical. The Captain hears them mention "World War One", which the Twelfth Doctor confirms, saddening the soldier as he felt the Great War would be the only conflict to reach such a scale, causing the Doctor to apologise: "spoilers". The First Doctor notices the Captain's unease, telling the soldier to sit down and orders the Twelfth Doctor to fetch some brandy.

Initially perplexed, the Twelfth Doctor does so, retrieving a glass. He hands it to the First Doctor who queries if he has had a glass, which his future self sarcastically confirms, given he has had 1500 years to do so. The First Doctor presses on, calling the Twelfth Doctor a nurse because he is "gentle", like a woman. The Twelfth Doctor tells his younger self that he can't say that, as he will be around many people who will dislike such a statement in the future. The First Doctor, seeing the Captain calm down, examines the new desktop theme, crticising how his successor has turned the "most powerful ship in the universe" into a "restaurant for the French". Before the Twelfth Doctor can reply, the First Doctor spies his older self's electric guitar, something the Twelfth Doctor tries to dismiss as his younger self surmises he has played it given it is the only thing that is not covered in dust. Regarding the dust, the First Doctor voices his disappointment in discovering that Polly must no longer be around to clean the TARDIS. Once more, the Twelfth Doctor tells his younger self to stop saying things like that.

Main cast

 * The Doctor - Peter Capaldi
 * The Doctor - David Bradley
 * Bill - Pearl Mackie
 * Clara - Jenna Coleman
 * Nardole - Matt Lucas
 * Voice of the Daleks - Nicholas Briggs

Guest cast

 * The Captain - Mark Gatiss
 * Helen Clay - Nikki Amuka-Bird
 * German Soldier - Toby Whithouse
 * Polly - Lily Travers
 * Ben - Jared Garfield

The Tenth Planet

 * The Doctor - William Hartnell
 * Polly - Anneke Wills
 * Ben - Michael Craze


 * And introducing Jodie Whittaker as the Doctor

The Tenth Planet

 * General Cutler - Robert Beatty
 * Krail - Reg Whitehead
 * Cyberman Voices - Roy Skelton, Peter Hawkins
 * The Doctor - Patrick Troughton

Invasion of the Dinosaurs

 * The Doctor - Jon Pertwee

Arc of Infinity

 * The Doctor - Peter Davison
 * Nyssa - Sarah Sutton

The Happiness Patrol

 * The Doctor - Sylvester McCoy

The Night of the Doctor

 * The Doctor - Paul McGann

The Day of the Doctor

 * The Doctor - John Hurt

The Parting of the Ways

 * The Doctor - Christopher Eccleston

The Waters of Mars

 * The Doctor - David Tennant

Journey's End

 * Davros - Julian Bleach

The Pandorica Opens

 * Cyberman Voice - Nicholas Briggs

Crew
to be added

Culture

 * British soldiers are fighting German soldiers.

Technology

 * The soldiers in World War I uses guns and rifles.
 * The Captain finds a VHS cassette inside the Twelfth Doctor's TARDIS.
 * The First Doctor finds the Twelfth Doctor's electric guitar.
 * The Testimony Foundation owns a spaceship which they used to capture the Twelfth Doctor's TARDIS.
 * The Twelfth Doctor says that Bill Potts was turned into a Cyberman.
 * The First Doctor is confused about the sonic screwdriver.
 * The First Doctor doesn't approve of the sonic sunglasses.
 * The First Doctor doesn't know what browser history is.
 * The Twelfth Doctor attempts to connect the glass woman via the sonic shades and TARDIS scanner to the TARDIS databanks.
 * He however finds that due to it being the First Doctor's TARDIS, there's hardly anything there, and needs a bigger one, and suggest the Matrix on Gallifrey.
 * Rusty scans the Twelfth Doctor.
 * The First Doctor picks up a Dalek eyestalk.
 * Rusty "gets naked" for the Twelfth Doctor by ejecting his gunstick.
 * The Twelfth Doctor mentions the Dalek hive mind.
 * The glass woman says that a perception filter will render her and the Doctors invisible.

Biology

 * Rusty scans the Twelfth Doctor, noticing his life signs are failing.
 * When the First and Twelfth Doctors shake hands, regeneration energy glows intensely from them.

Food and beverages

 * A soldier is seen drinking tea.
 * Both Doctors has brandy inside one of the roundels in each of their TARDISes.
 * The soldiers are drinking beer.
 * The Twelfth Doctor tells himself to never eat pears.

Sport

 * The World War I soldiers are playing football.

People

 * Captain Lethbridge-Stewart and a German soldier both had no desire to kill the other, wishing the other spoke English and German, respectively.
 * Everyone except The Doctors, the Captain and the glass avatars freeze on the spot when time freezes.
 * The First Doctor remarks that it's a shame Polly Wright isn't around, saying that Twelve's TARDIS is in need of cleaning.
 * Bill Potts returns to the Doctor's side.
 * Bill tells that Heather, the sentient oil girl, came back for her.
 * Bill comes to realise the two Doctors are the same man.
 * Bill asks the First Doctor about his reason to run away.
 * The Doctor finds Rusty inside a tower in Villengard.
 * Rusty projects a video of a Professor Helen Clay.
 * Clara Oswald appears to the Twelfth Doctor and his memories of her returns.
 * Nardole returns to say goodbye to the Doctor.
 * Nardole reveals that he used to have nipples made of glass, and that he has invisible hair.

Species

 * The First Doctor ask the Twelfth Doctor if he is a Time Lord.
 * The Twelfth Doctor says the First Doctor was "weak as a kitten".
 * A bird freezes mid-air.
 * Glass avatars, part of the Testimony, are creatures with ability to freeze time and extract memories and personality of a dying person so they will remain forever.
 * The TARDIS team encounters Kaled mutants which has mutated even further.
 * The Captain first thinks they are rats, saying he is used to those.

Locations

 * The First Doctor has just left the Snowcap base in search for his TARDIS.
 * The Doctors meet eachother at the South Pole.
 * Captain Lethbridge-Stewart is fighting during World War I in Ypres, in 1914.
 * The First Doctor says to the glass avatar that it is on Earth, a "level 5 civilisation".
 * The Doctors and the Captain are taken to the Chamber of the Dead inside a Testimony spaceship.
 * Gallifrey is mentioned.
 * The TARDIS team visits Villengard.
 * The moon is in the process of destruction.
 * Rusty projects a video of Professor Helen Clay from the University of New Earth in the year 5,000,000,012.

Events

 * Northern lights appear in the sky above the South Pole.
 * The glass avatars freezes time at the South Pole in 1986 only minutes after the battle at Snowcap base.
 * The glass avatars freezes time during World War I in Ypres 1914 just before the Christmas truce.
 * When the glass avatars misplaced the Captain they called it a timeline error.
 * The one and only ever Christmas truce is held during the 1914 Ypres battle during World War I.
 * The Captain says he told his wife he'd be home for Christmas.
 * A Christmas Armistice happened during World War I, on 25 December 1914.

The Doctor

 * Due to the timelines being out-of-sync, the First Doctor forgets trying not to regenerate and the Twelfth Doctor does not remember it.
 * The First Doctor's face is, according to Twelve, "all over the place" as a result of him trying to hold off his regeneration. This also results in him regaining the strength he had lost during his fight with the Cybermen on Snowcap.
 * The First Doctor doesn't know who his future self is, and keeps denying it even after the Twelfth Doctor reveals himself to him. Only after the Testimony shows him snippets of his future does he finally give in and accept it.
 * The First Doctor first thinks the Twelfth Doctor is another Time Lord trying to capture him for running away with a TARDIS.
 * The First Doctor later attempts to make the Captain understand better, saying that the Twelfth Doctor is his nurse, however improbable due to him being a man.
 * The First Doctor tells the Twelfth Doctor that he assumed he would get a younger appearance after regenerating.
 * The Twelfth Doctor tells the First Doctor that between those two there are "a few false starts".
 * The Doctor is called "The Destroyer of Worlds", "The Oncoming Storm", "The Shadow of the Valeyard", "The Beast of Trenzalore", "The Butcher of Skull Moon", "The Destroyer of Skaro" and "the Doctor of War".
 * The Twelfth Doctor calls the First Doctor Mary Berry.
 * The Twelfth Doctor attempts to distract the First Doctor with the blinking lights on his astral map after his unfavourable remark to Bill about Twelve needing her for cleaning his TARDIS.
 * The First Doctor admits to being afraid of regeneration.
 * The First Doctor says he travel around to get a question answered about why there is good and evil in the universe and how the balance is held between the two.
 * The Twelfth Doctor admits to only pretending to like Bill's rug.
 * The First Doctor regenerates into the Second Doctor.
 * The Twelfth Doctor regenerates into the Thirteenth Doctor.
 * The Twelfth Doctor's ring falls off of the Thirteenth Doctor's hand, as it doesn't fit on.

TARDIS

 * The First Doctor calls the TARDIS "the Ship".
 * The First Doctor remarks on the exterior changes; the windows being the wrong size, the blue colouring having changed and its size having expanded.
 * The Twelfth Doctor says its size is because of all those years having been "bigger on the inside".
 * The First Doctor disaproves of the Twelfth Doctor's TARDIS, saying about the console room that it is "the flight deck of the most powerful space time machine in the known universe, not a restaurant for the French".
 * The Twelfth Doctor still has blackboards in his TARDIS.
 * Bill remarks the windows are the wrong size.
 * The First Doctor's TARDIS has a name tag on the console that says Bernard Wilkie.
 * The Cloister Bell rings in the Twelfth Doctor's TARDIS, before he regenerates.

Story notes

 * The cinematic release featured two documentaries: The End of an Era, a retrospective and behind-the-scenes look, before the showing, and Twice Upon a Time at the end, an in-depth look into the making of this Christmas special.
 * Twice Upon a Time features a "Previously on Doctor Who..." sequence saying "709 Episodes ago" and showing footage from TV: The Tenth Planet with William Hartnell, Anneke Wills and Michael Craze as the First Doctor, Polly Wright and Ben Jackson respectively. The footage slowly morphs Hartnell into David Bradley, and Lily Travers and Jared Garfield then appear as Polly and Ben, who then proceed to reenact the missing bits on Snowcap from said episode.
 * Rachel Talalay had actually shot a number of scenes from The Tenth Planet, with a complete set reconstruction, recast characters, and the series 10 Mondasian Cybermen. She said in an interview that she tried to make both the set, and the cinematography, as accurate to the original scenes, and original production values, as much as possible. She did admit to having fun with some parts of the set in particular, and embellishing with a "60s feel", and noted that some details of the action had to be invented. These scene recreations were not used in the final edit of Twice Upon a Time; the original footage was used in its place. However, these specially shot re-tellings were shown in the cinema feature DOC: Twice Upon a Time.
 * The First and Twelfth Doctors meeting also features the scene acted out differently in TV: The Doctor Falls and Twice Upon a Time, similar to how episodes in the 60s reshooting the prior scene’s cliffhanger, often resulting in little differences. The same situation also occured when TV: The Runaway Bride followed the TV: Doomsday cliffhanger 11 years prior.
 * The recreated Hartnell TARDIS interior set actually included props from the original set in the 1960s. As told by Mark Gatiss, these included the brass pillars, which are indeed the brass pillars from way back in 1963. Other aspects of the set were recreated.
 * The Twelfth Doctor's regeneration has a new visual effect, and even sound effect, to those used previously in the BBC Wales series. Resembling slow travelling, elongated golden lightning. It is the first regeneration for the Doctor, since the Seventh Doctor's in 1996, to use an effect which is visually distinct from that seen in The Parting of the Ways, with the Ninth Doctor up to The Doctor Falls.
 * Toby Whithouse, with his performance here as the German soldier, is the second person to be involved in the BBC Wales series as both actor and writer, following Mark Gatiss.
 * Given that the First Doctor is amazed and confused about the Twelfth Doctor's sonic screwdriver, this story makes PROSE: Venusian Lullaby non-canon, as it features the screwdriver used by the First Doctor.
 * This is the third consecutive regeneration story to include the word "time" in the title, following The End of Time and The Time of the Doctor.
 * This is the second multi-Doctor story to feature a regeneration, and the first in which it is the current incarnation of the Doctor who is shown to regenerate. (TV: The Day of the Doctor)
 * Nikki Amuka-Bird is credited as "Helen Clay" on-screen, and as "The Glass Woman" in Radio Times.
 * When Bill's glass avatar and the Doctor talk and she restores his memories of Clara Oswald, the music from The Husbands of River Song when the Doctor and River were on a date before the Singing Towers of Darillium plays.
 * Nardole having invisible hair was already jokingly stated by Steven Moffat in a BBC post episode interview. There he also stated that Nardole was the only one able to see it through his glasses and that it was purple. Matt Lucas had been aware of the invisible hair, but not the glass nipples.
 * The Twelfth Doctor's final lines about his name and how humans would understand it are directly taken from a theory Capaldi stated at a press screening interview of The Pilot.
 * The Twelfth Doctor's line "never, ever eat pears" is a tweaked version of the one in a hidden scene in TV: Human Nature by the Tenth Doctor. It was also said in TV: Hell Bent by the Twelfth Doctor.
 * This is the first regeneration story in the revived series without any direct connections to the Last Great Time War.
 * This is the third televised multi-Doctor story, after TV: The Two Doctors and Time Crash, that is not an anniversary special. It's also the first televised Christmas special to also be a multi-Doctor story.
 * Time Crash was also written by Steven Moffat.

Ratings

 * 5.70 million (UK overnight)

Filming locations
to be added

Production errors
to be added

Continuity

 * The Twelfth Doctor mentions Bill's Cyber-conversion and apparent death, (TV: World Enough and Time / The Doctor Falls) and Bill explains how Heather saved her. (TV: The Doctor Falls)
 * The Twelfth Doctor declares everyone is important. The Eleventh Doctor thought so too. (TV: A Christmas Carol) The Ninth Doctor previously claimed an "ordinary man" was the most important thing in creation, and expressed shock when someone said they weren't important. (TV: Father's Day)
 * The Twelfth Doctor indirectly references that the TARDIS, as time passes, inevitably suffers from a size leak. (TV: The Name of the Doctor)
 * Bill reminds the Twelfth Doctor about Heather and the sentient oil. (TV: The Pilot)
 * Despite remembering that Heather saved her, Bill is unaware of Heather's current whereabouts, implying her own transformation from human to sentient oil is considered her 'death' by the Testimony, as this is the point at which her memories run out. (TV: The Doctor Falls)
 * The Twelfth Doctor tells the Testimony that the Earth is defended and comments on how "that almost never works" when the Testimony retreats briefly. (TV: The Eleventh Hour et. al) The First Doctor is surprised by this, causing the Twelfth Doctor to remember that he is "early days."
 * The Twelfth Doctor informs the First Doctor he must remember where he parked, having got in the wrong TARDIS. The Eleventh Doctor would later repeat this sentiment to himself when on a date with River Song. (TV: Last Night)
 * The Twelfth Doctor is well aware of the consequences that would arise from the premature death of his predecessor and the resulting erasure of his deeds. (TV: Turn Left, The Name of the Doctor)
 * The First Doctor remarks on his successor playing the electric guitar. (TV: The Magician's Apprentice, Hell Bent, The Pilot)
 * The First Doctor declares he will not change. The Twelfth Doctor said the same thing. (TV: The Doctor Falls)
 * The Twelfth Doctor visits the South Pole as he nears regeneration. He was there, also near regeneration, in his first incarnation. (TV: The Tenth Planet)
 * The Doctor was present for the Christmas truce of 1914 earlier in his first incarnation (PROSE: The Little Drummer Boy) as well as during his fifth (PROSE: Never Seen Cairo) and ninth incarnations. (COMIC: The Forgotten)
 * The First Doctor is unfamiliar with the sonic screwdriver. (TV: Fury from the Deep, et. al)
 * The Twelfth Doctor goes to the abandoned weapon forges of Villengard, to find an old ally. The Ninth Doctor previously told Jack Harkness that he had once visited Villengard, when the main reactor went critical, and vapourised everything. Jack's sonic blaster was made here. (TV: The Doctor Dances)
 * The First Doctor recognises what the Twelfth Doctor knows to be a Kaled mutant on Villengard, having encountered such mutations on his first visit to Skaro. (TV: The Daleks)
 * The First Doctor explains why he ran away from Gallifrey. (TV: Heaven Sent)
 * The Twelfth Doctor meets Rusty again. (TV: Into the Dalek)
 * Rusty calls the Twelfth Doctor a "good Dalek". He did so before. (TV: Into the Dalek, Death in Heaven)
 * The Doctor mentions that Earth is a "level 5 civilisation". (TV: City of Death, Voyage of the Damned, The Eleventh Hour)
 * Helen Clay is from New Earth. She founded the Testimony Foundation in 5,000,000,012, just a few years before the Tenth Doctor and Rose first visited. (TV: New Earth, Gridlock)
 * The Doctor holds a Dalek eyestalk. (TV: The Wedding of River Song, The Time of the Doctor)
 * The Doctor is referred to as "the Doctor of War". (TV: A Good Man Goes to War, The Name of the Doctor, The Day of the Doctor, Hell Bent) The Testimony also calls the Doctor by other titles associated with conflict, as did the Daleks, (TV: The Parting of the Ways, Asylum of the Daleks, PROSE: Engines of War) and the Great Intelligence. (TV: The Name of the Doctor)
 * The Doctor meets his first incarnation. He has met him before and also struggles to remember the encounter. (TV: The Three Doctors, The Five Doctors, The Day of the Doctor)
 * The Twelfth Doctor tells the First Doctor they have an advantage as "this time there's two of us". The Eleventh Doctor made a similar remark when talking to his Tenth and War incarnations. (TV: The Day of the Doctor)
 * The Doctor once more pilots a previous incarnation's TARDIS. (TV: The Day of the Doctor)
 * The Doctors banter about their differences. (TV: Time Crash, The Day of the Doctor)
 * The First Doctor still calls the TARDIS "the Ship". (TV: The Daleks, The Sensorites, PROSE: Time and Relative)
 * The Twelfth Doctor favourably remarks about the desktop theme of the First Doctor's TARDIS. The Tenth and Eleventh Doctors did the same regarding the War Doctor's TARDIS. Likewise, the First Doctor criticises the Twelfth Doctor's desktop theme just as the Tenth did the Eleventh's. (TV: The Day of the Doctor)
 * The events of The Tenth Planet are referred to.
 * The Testimony shows the Doctors the amount of times they have become associated with wars. Notably, events from The Waters of Mars, Journey's End, The Day of the Doctor and The Name of the Doctor.
 * The Testimony also show snippets of the Doctor's previous incarnations: the Third Doctor (TV: Invasion of the Dinosaurs), the Fifth Doctor, (TV: Arc of Infinity), the Seventh Doctor, (TV: The Happiness Patrol), the Eighth Doctor, (TV: The Night of the Doctor) the War Doctor, (TV: The Day of the Doctor), and the Ninth Doctor. (TV: The Parting of the Ways)
 * The Testimony also shows Davros. (TV: Journey's End)
 * The Eleventh Doctor's conflict at Trenzalore is mentioned by the Testimony, (TV: The Time of the Doctor) as is the Pandorica, (TV: The Pandorica Opens) the Last Great Time War, (TV: Dalek et al.) and the destruction of Skaro. (TV: Remembrance of the Daleks)
 * The Testimony tells the Doctor that he is in the shadow of the Valeyard. (TV: The Mysterious Planet, Mindwarp, Terror of the Vervoids, The Ultimate Foe) The Valeyard had previously caused the Sixth Doctor to regenerate. (AUDIO: The End of the Line, The Red House, Stage Fright, The Brink of Death)
 * The Twelfth Doctor repeats River Song's favourite phrase of "spoilers" to his predecessor. After the First Doctor is surprised to learn that Earth is defended, he calls the First Doctor "early days," the same phrase used by River to refer to the Tenth Doctor. (TV: Silence in the Library)
 * The Twelfth Doctor remembers his promise to "never be cruel or cowardly. Never give up, never give in." (TV: The Day of the Doctor)
 * The Twelfth Doctor uses his astral map (TV: The Web Planet) and the TARDIS data-bank (TV: Castrovalva)
 * The Twelfth Doctor mentions the Daleks' Pathweb. (TV: Asylum of the Daleks, The Time of the Doctor)
 * The Twelfth Doctor once more references River's words from her diary. (TV: Extremis, The Doctor Falls)
 * The Doctors promise to check up on the family of Lethbridge-Stewart. (TV: The Web of Fear, et al.)
 * The Twelfth Doctor regains his memories of Clara. (TV: Hell Bent, For Tonight We Might Die, The Pilot)
 * Regeneration energy has previously dealt severe damage to the Doctor's TARDIS, leaving it in disarray for the successor. (TV: The End of Time)
 * The Twelfth Doctor mentions Bill's Christmas gift of a rug. (TV: The Pilot)
 * The final shot of the Twelfth Doctor is a shot of his eyes, echoing his first appearance. (TV: The Day of the Doctor) His ring also falls to the ground shortly after regenerating, mirroring when his predecessor's bow tie fell to the ground as he regenerated, (TV: The Time of the Doctor) and recalls how the First Doctor's ring no longer fitted the Second Doctor's finger, leading him to discard it. (TV: The Power of the Daleks)
 * The Twelfth Doctor says "Doctor, I let you go" before regenerating. Before he blew up the Cybermen on the Mondas Colony Ship, he previously told himself to "let it go". (TV: The Doctor Falls) In contrast, as his tenth incarnation regenerated, he had said "I don't want to go". (TV: The End of Time)
 * The First Doctor chides Bill for swearing, echoing how he did similar with his own granddaughter Susan when she made crude remarks. (TV: An Unearthly Child)
 * The Twelfth Doctor recalls where the alcohol is stored in the TARDIS, having seen River access it. (TV: The Husbands of River Song)
 * The Twelfth Doctor tells his successor not to eat pears. The Seventh and Twelfth Doctors previously expressed a distaste for pears. (PROSE: Human Nature, TV: Hell Bent) Though the human John Smith was seen eating a pear in TV: Human Nature, footage which is used, sped up, in that same episode, included the Tenth Doctor himself warning Martha not to let his human counterpart eat pears.
 * The Thirteenth Doctor expresses delight when she sees her new reflection in the TARDIS view screen. The Twelfth Doctor previously expressed approval when Clara suggested he could become female. (TV: Death in Heaven) The Eleventh Doctor also expressed enjoyment at being a woman when he switched bodies with Amy Pond. The Doctor noted that "Time Lords don't really worry that much about what sex they are." (COMIC: Body Snatched) He also mistakenly thought he had regenerated into a woman shortly after his own regeneration. (TV: The End of Time)
 * The TARDIS doors fly open after the Doctor has regenerated and the TARDIS is malfunctioning. (TV: The End of Time, The Eleventh Hour)
 * Shortly before regenerating, the Twelfth Doctor salutes Captain Archibald Hamish Lethbridge-Stewart, something that the Brigadier always wanted the Doctor to do. (TV: Death in Heaven)
 * The glass avatar of Bill Potts mentions that the Doctor "serves with the pleasure of the human race". (TV: Thin Ice)
 * Nardole telling the Doctor that "if he dies then everyone in the universe would go cold", resembles the Eleventh Doctor telling his old companions Sarah Jane and Jo Grant that "if he died then he thinks the whole universe might just shiver". (TV: Death of the Doctor)
 * The Twelfth Doctor says "Snap" to the First Doctor when revealing that he's his future self, in a similar fashion to when the Sixth and Second Doctors met. (TV: The Two Doctors) The Tenth Doctor also did the same when comparing how similar he was to the Fifth Doctor. (TV: Time Crash) "Snap" was also a phrase River Song told the Tenth Doctor after showing him her screwdriver. (TV: Silence in the Library)
 * The First Doctor threatens Bill with a "jolly good smacked bottom". He told Susan Foreman she also deserved "a jolly good smacked bottom". (TV: The Dalek Invasion of Earth)
 * The Twelfth Doctor wears his sonic sunglasses again. (TV: The Magician's Apprentice et al)
 * The Twelfth Doctor gets concerned when the First Doctor asks what browser history. is. He previously warned Osgood and Bill not to look at his browsing history. (TV: The Zygon Inversion, Smile)

DVD releases
to be added

Blu-ray releases
to be added

Digital releases
to be added