Twice Upon a Time (TV story)

Twice Upon a Time was the 2017 Christmas Special of Doctor Who. It was the show's thirteenth Christmas special since its revival and the fourth starring Capaldi as the Doctor. It was also the final chronological appearance of Peter Capaldi as the Twelfth Doctor and introduced Jodie Whittaker as the Thirteenth Doctor in its closing moments.

After a surprise appearance at the end of TV: The Doctor Falls, the First Doctor was an integral part of the special's narrative. Here, he was portrayed by David Bradley, who had previously played the original actor William Hartnell in An Adventure in Space and Time. Archival footage from The Tenth Planet was used to bookmark the First Doctor's place in this story.

Ben Jackson and Polly Wright made a brief re-reappearance in the programme for the first time since their exit in 1967's The Faceless Ones. Bill Potts joined the two Doctors as a Testimony glass avatar. Past companions Nardole and Clara Oswald made similar brief appearances. Rusty, from Capaldi's second episode, Into the Dalek, made his second appearance in Villengard, a location mentioned only once before on television, in Steven Moffat's second television story, 2005's The Doctor Dances.

In early 2018, Murray Gold announced his departure from Doctor Who after twelve years at the Gallifrey One convention, confirming that Twice Upon A Time was his last episode. It was also the final story with Steven Moffat as head writer and the first to include any part written and overseen by incoming lead writer, Chris Chibnall.

While not known at the time, this would also turn out to be the last consecutive Christmas special of the revived series, breaking the streak that began with The Christmas Invasion in 2005. The show would forgo its usual Christmas special in 2018 in favour of a New Year Special at the beginning of 2019.

Synopsis
As the Doctor nears regeneration, he stumbles on his original self, also refusing to change. It takes a captain, a glass avatar and a familiar face to convince the Doctors the universe still needs them.

Plot
The First Doctor nears his death after an encounter with the Cybermen from Mondas. He is freed from his restraints by his companions Ben and Polly, now that the threat is over. However, he is weak, his body beginning to regenerate. Refusing to give in, the Doctor leaves and wanders the South Pole, mumbling to himself that he will not change. Nearing his TARDIS, he hears a voice in the distance yelling in defiance. Seeing a figure kneeling outside his TARDIS, the Doctor asks who the man is. The man claims to be the Doctor. With a smug sense of superiority, the First Doctor tells the man that's not true; "you may be a doctor, but I am the the doctor. The original you might say."

The scene transitions to the Twelfth Doctor as he gets to his feet, shocked to see his past self. The Doctor is delighted at meeting his first incarnation but realises that he is refusing to regenerate; he panics as he doesn't remember refusing to to regenerate the first time or meeting himself. The First Doctor is confused and dismisses his future self as just another Time Lord; he asks if he has come to take back "the ship". The Twelfth Doctor laughs in amusement that his original incarnation still calls the TARDIS "the ship".

The First Doctor then asks what the Twelfth has done to the TARDIS, as its bigger than it used to be. The Twelfth Doctor tells him its from "all those years of being bigger on the inside. You try keeping your tummy tucked in that long", defending his beloved vessel. The Twelfth Doctor questions why his younger self is refusing to regenerate, fearing the consequences the decision might cause. However, the snow suddenly freezes in midair, capturing the pair's attention, as a man dressed in World War I attire approaches them. He seems disorientated and asks the pair if either of them are a doctor. Exchanging a glance with the First Doctor, the Twelfth asks if the man is making some sort of joke.

On Christmas Day 1914, the soldier, a captain in the British Expeditionary Force, lies in a shell hole on the battlefield in Ypres. His revolver is aimed at a German soldier, who also has his sidearm trained on the Captain. Both men realise they will kill each other in self-defence, 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 around him. As he exits the hole, a bright light suddenly flashes and encapsulates him. He collapses to the ground in the Antarctic and 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, and the two Doctors approach it. The First Doctor orders the portal to reveal itself, while the Twelfth Doctor declares that the planet is protected, to the confusion of his first incarnation. 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, and the Twelfth Doctor coaxes him to follow. As they enter, the First Doctor is shocked by the changes to the TARDIS interior, while the Captain is shocked by the room's size. The Twelfth Doctor pulls the monitor over to the First Doctor, telling him his TARDIS is 70 feet away; he then tells him to always remember where he parks as it gets important later.

The First Doctor demands his identity, to which the Twelfth says not to be an idiot as they always know when they meet each other. However, the First Doctor states he does not know, prompting the Twelfth Doctor to show him his hand glowing with regeneration energy. The First Doctor wonders if the Twelfth is his next incarnation, to which the Twelfth says is not the case; he'll become him eventually, "after a few false starts" the Twelfth says with a grin.. The First Doctor mutters that he thought he would get younger when he regenerated, to which the Twelfth snaps that he is younger than the First Doctor. The Captain interrupts, stating he does not understand what's happening, while examining a VHS tape. The Twelfth Doctor says that's understandable for a solider from World War I being chased through frozen time by an unknown force. Disturbed upon hearing the Great War being called the first world war, the Captain asks the Doctor what he's talking about; the Doctor simply tells him "spoilers".

The First Doctor notices the Captain's unease, telling the soldier to sit down and orders the Twelfth Doctor to fetch some brandy. He does so, and the First Doctor notices that there is less in the glass than he remembered. The Twelfth Doctor tells him that it's been "rock and roll" over the last 1800 years, so he's entitled to have a glass to steady his nerves. The First Doctor examines the control room, noting that the Twelfth no longer has Polly around since it's so dusty; the Twelfth Doctor begs him to stop saying things like that as it is insulting to women. The First Doctor then notices the Twelfth's electric guitar and amp, which the Doctor tries to pass off as something a companion left behind. However, the First Doctor notes that it is the only thing in the control room that's not covered in dust, making it the Twelfth Doctor's possession.

Suddenly, the TARDIS shakes violently, prompting the Twelfth Doctor to attempt to take off, only to find that the engines are unresponsive. Looking upwards out of the TARDIS doors, he sees large metal claws pulling the TARDIS upwards. Once the TARDIS has stopped moving, the Twelfth Doctor instructs his predecessor to confront their captors. A humanoid glass figure seated at the top of a staircase speaks to the Doctor, addressing him as "The Doctor of War." The figure requests the return of the Captain in exchange for a friend of the Doctor. Much to the confusion of the First Doctor, Bill then exits one of the corridors. The Twelfth Doctor quickly exits the TARDIS to hug her, before slowly pulling away, drawing his sonic screwdriver, and scanning Bill. To her surprise, the Twelfth Doctor demands that she proves herself to be real, as he believes Bill to have sacrificed herself to defeat the Cybermen. Bill insists she is her real self, explaining how Heather had saved her. The Twelfth Doctor asks what happened after Heather saved her and where Heather is now. Bill struggles for an answer.

The Twelfth Doctor decides to confront his captor directly and runs up the staircase. He is slowly followed by his younger self, who questions the need to move so fast. They find an empty chair with several panels behind it. The First Doctor takes to observing the panels, commenting on their extraterrestrial origin. He turns to his older self, his eyeglass dropping from his eye as he notices the Twelfth Doctor wearing sunglasses to scan the panels. He is utterly perplexed by the idea that the sunglasses are sonic. They turn to the chair, where a woman made completely of glass materialises. While the Twelfth Doctor scans the panels with his sonic screwdriver, the original Doctor looks more closely. He berates the Twelfth Doctor for not noticing the asymmetrical features of the woman's face, blaming his use of the sunglasses instead of his eyes. The Twelfth Doctor shyly agrees, and his younger self throws the sunglasses to the ground.

The Testimony proceeds to explain its desire for the Captain back in exchange for Bill, insisting the Doctors return him to them. The Captain, having heard the conversation, exits the TARDIS. He declares they let him be taken, not wanting any harm to come to Bill because of his cowardice. However, the Twelfth Doctor disagrees. Picking up his sonic sunglasses, he races down the stairs, remarking he will find out what the Testimony's purpose is and he will stop it if he doesn't like it. The First Doctor once more berates him for revealing his plan, stating he hasn't a clue who his future self is supposed to be. The Testimony, in an effort to ruffle the First Doctor, reveals the history he is destined to create as a "Doctor of War" by showing how his future incarnations will engage in various battles, which perplexes him.

The Twelfth Doctor presses on with his plan to escape, using the sonic screwdriver to activate the crane which pulled the TARDIS up to take them down, the team jumping onto the chains to escape as well. The Testimony refuses to let them escape, halting the chains and pulling the TARDIS back into the ship. However, as they are a safe distance from the ground, the Twelfth Doctor instructs everyone to jump to the ground.Bill expresses concern about what they will do without the TARDIS but the Doctors reveal they have another, younger version, at their disposal.

They head to the First Doctor's TARDIS, swiftly entering. As she does so, Bill comments on how much smaller it is, causing the Twelfth Doctor to briefly observe the windows before insisting his younger self take off into "deep space, anywhere". While in flight, the First Doctor insists Bill clean the TARDIS only for the Twelfth Doctor to distract him by pointing out the astral map. The Twelfth Doctor uses the sonic sunglasses to access the data banks noting what his predecessor said earlier before placing the shades on the First Doctor. Befuddled, the Doctor asks why he must wear them only for his successor to remark on how much he loves it before taking them back.

The Twelfth Doctor quickly realises the databanks are rather scarce, given the TARDIS is still young. To compensate, he lands the TARDIS at the centre of the universe, Villengard, impressing his younger self at how easily he did such. While searching the area, the Captain is attacked by a creature with multiple little green legs but they save him, both Doctors recognising the creature as a Kaled mutant. The Twelfth Doctor insists Bill wait in the First Doctor's TARDIS to look after the Captain, but Bill blasts him by calling him a "stupid bloody arse", leading to the First Doctor to threaten her with a "jolly good smacked bottom", much to the Twelfth Doctor's embarrassment. Bill tells the Twelfth Doctor to come back alive, to which he replies; "Be here when I do."

The First Doctor fetches the Captain another brandy, realising that was why his successor was missing some. He soon leaves to join his older self, while Bill looks after the Captain, but when she grabs the brandy, her hand is revealed to be glass. While outside on Villengard, the two Doctors rest, discussing their respective regenerations. The Twelfth Doctor remarks they have yet to change because they are in a "state of grace", though he himself finds his body starting to deteriorate. He asks why his younger self halted the regenerative process, unable to remember himself. The First Doctor admits he is "afraid" of regenerating and doesn't like to admit that to anyone. The Twelfth Doctor remarks that technically he still hasn't. Before the Twelfth Doctor can answer the question himself, they suddenly come under attack from a creature in a nearby tower, and only stops when the Twelfth Doctor asks the creature to scan him, announcing; "I'm dying!", before going into the tower. He tells his younger self to wait outside to which he begrudgingly agrees.

The First Doctor observes the area, finding the remains of a Dalek casing. He inspects it as Bill sneaks up on him and begins asking why he ran away. The Doctor insists she return to the Captain but Bill insists, asking what he was running to. Interested by the question, the First Doctor responds that he left to discover why good always defeats evil. Bill suggests it might be because of "some bloke" though the First Doctor doubts such. Bill hugs him, thanking him for his efforts. However, when she pulls away, the First Doctor realises she is a glass avatar.

Alone, the Twelfth Doctor meets with the rogue Dalek Rusty, who has taken refuge from the other Daleks hunting it. He remarks on how he is a "good Dalek" though Rusty continues to fire at the Doctor. The Twelfth Doctor decides to come to a truce to which Rusty agrees by disarming his gunstick. Given access to the Dalek Hivemind, the Doctor learns that the pilot and its ship, known as Testimony, were created on New Earth, designed to extract people from their timelines at the moment of their death, and archive their memories into glass avatars. "Bill" is one such avatar, created from her memories. Time suddenly freezes again, the First Doctor and Bill entering the room. Although disappointed at seeing no evil to fight, the Doctors agree to return the Captain to his timeline.

Upon doing so, the Captain asks the Doctors to keep an eye on his family, introducing himself as Archibald Hamish Lethbridge-Stewart, an ancestor of the Doctor's close friend the Brigadier. The First Doctor assures he will do so, the Twelfth Doctor confirming they did. The Testimony remarks the Captain will not remember the incident and a perception filter will ensure the Doctors are not noticed once time restarts. As time resumes, the Doctors watch as soldiers on both sides begin singing "Silent Night" in their respective languages. Soon, each side disarms their weapons and enters the battlefield, leading the Captain and the German soldier to withdraw their weapons and make peace. The Twelfth Doctor explains to the First that he deliberately shifted the Captain's timeline to the start of the Christmas truce, to ensure his life would be spared. The First Doctor is amazed at the fact that his successor altered time to save both men, leading the Twelfth Doctor to describe the truce as a miracle; the only truce of its kind in history. How everyone at war stopped fighting and was kind to each other. He also explains how "generally the universe isn't a fairy tale, but that's where we come in" and that "it never hurts, a couple fewer dead men on a battlefield." This causes the First Doctor to finally understand what being the "Doctor of War" really means.

With the Captain saved, the two Doctors observe the truce for a time and eventually shake hands. As they do, both Doctors' hands start glowing with regeneration energy. The First Doctor declares that he is finally ready, but the Twelfth Doctor states that his predecessor will have to find out what the Twelfth Doctor decides to do "the long way round." The two Doctors bid each other a fond farewell. The First Doctor returns to his TARDIS and sets the controls to return him to the South Pole in 1986. Overcome by his regeneration, the First Doctor declares "here we go, the long way round" and collapses as his TARDIS returns to the South Pole. Shortly afterwards, the First Doctor regenerates into a much younger second incarnation.

As the soldiers play football, draw and spend the day together, the Twelfth Doctor watches on, saluting the Captain - who briefly bypasses the perception filter, but dismisses him and returns to his trench. Now alone with Bill's avatar, the Doctor adamantly contends she is not the genuine Bill, only for her avatar to explain that memories are what define a person. To help him understand, the avatar restores the Doctor's memories of Clara Oswald then turns into her. Clara greets the "stupid old man", telling him that forgetting her was rude. The Doctor laughs with joy that she's back in his head. Clara reverts back to Bill, who asks the Doctor if he now understands the importance of memory. They are joined by Nardole's avatar who joins Bill in asking the Doctor not to allow himself to die. The Doctor refuses to give the avatars testimony of his life, stating he must be left alone to face his end. They respect his wish and leave after he embraces them both.

The Doctor returns to the TARDIS. He laments how the more he saves the universe, the more it seems to be in need of saving. He briefly argues with the TARDIS, conceding that "they would get it all wrong without me" as he decides to regenerate, saying "One more lifetime won't kill anyone. Well, except me", but not before relaying advice to his next incarnation. He collapses as he does so but manages to return to his feet. Finally ready, the Twelfth Doctor utters his last words "Doctor... I let you go". With that, he takes a last quick gasp for air before he regenerates, the process so violent that the energy ignites the console room as his face and piercing blue eyes start changing, morphing into a younger-looking and rounder one with hazel eyes.

Smoke fills the console room and the Twelfth Doctor's wedding ring falls from a finger it no longer fits. Once the smoke clears, the Thirteenth Doctor stands in silence for a moment, before grabbing one of the console's monitors. Using its reflective surface, she studies her new appearance in stunned wonderment, realising that she has now become female for the first time. Her face cracks into a wide, ecstatic grin as she as exclaims "Oh, brilliant!" at the discovery. Excited, she presses a button on the console. Upon doing so, the TARDIS suddenly suffers multiple failures and, hanging thousands of feet above the Earth, sharply jerks causing the doors to face the ground. The Doctor tries to hang on but the doors open threatening to suck her out. As the Doctor grabs the console, the ship continues to shake, sending all of her books and papers flying out the open doors. The Doctor tries to close the doors, but the time rotor shatters and the panel she's holding onto breaks away, sending her falling out of the ship. In freefall towards the Earth below, the Doctor watches the inside of her TARDIS become swamped by flames before vanishing from sight as she plummets through the clouds...

Cast

 * The Doctor - Peter Capaldi
 * The Doctor - David Bradley
 * Bill - Pearl Mackie
 * The Captain - Mark Gatiss
 * Clara - Jenna Coleman
 * Nardole - Matt Lucas
 * Helen Clay - Nikki Amuka-Bird
 * German Soldier - Toby Whithouse
 * Polly - Lily Travers
 * Ben - Jared Garfield
 * Voice of the Daleks - Nicholas Briggs

The Tenth Planet

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


 * And introducing Jodie Whittaker as the Doctor

Uncredited crew

 * Writer - Chris Chibnall
 * VFX Supervisor - Murray Barber
 * CG Supervisor - Dominic Alderson
 * VFX Producer - Louise Hastings

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 sunglasses 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 suggests 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 to the soldiers on the battlefield.

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 have Aldebaran brandy in a compartment behind one of the roundels in each of their TARDISes.
 * The soldiers are drinking beer.
 * The Twelfth Doctor tells his future self to never eat pears.

Art

 * A soldier is seen drawing a picture.

Sport

 * The World War I soldiers are playing football.

People

 * Captain Lethbridge-Stewart and a German soldier both have no desire to kill the other, wishing the other spoke English and German, respectively.
 * The First Doctor remarks that it's a shame Polly Wright isn't around, saying that the Twelfth Doctor's TARDIS is in dire need of dusting.
 * Bill tells the Doctor about Heather, the sentient oil girl, coming back for her, to which the Twelfth Doctor notes "how romantic".
 * Bill asks the First Doctor about his reason for running away from Gallifrey.
 * 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 return.
 * Nardole returns to say goodbye to the Doctor.
 * Nardole reveals that he had nipples made of glass and that he has invisible hair.

Species

 * The First Doctor asks the Twelfth Doctor, not realising he's talking to his future self, if he is another Time Lord.
 * The Twelfth Doctor says the First Doctor was "weak as a kitten" when he started regenerating.
 * A bird freezes mid-air.
 * Glass avatars, part of the Testimony, are creatures with the 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 have 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 each other 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 travels to the centre of the universe.
 * The TARDIS team visits Villengard. The moon is in the process of destruction.
 * The Twelfth Doctor calls Villengard's Weapon Forges "The Nightmare of the Seven Galaxies".
 * The Captain says that his death would be a big shock for everyone back in Cromer.
 * Rusty projects a video of Professor Helen Clay from the University of New Earth on New Earth in the year 5,000,000,012.

Events

 * The aurora australis appears in the sky above the South Pole.
 * The glass avatars freeze time at the South Pole in 1986 only minutes after the battle at Snowcap base.
 * The glass avatars freeze time during World War I in Ypres 1914 just before the Christmas truce.
 * When the glass avatars misplace the Captain they call it a timeline error.
 * The Christmas truce is held on the World War I battlefield of Ypres on 25 December, 1914.
 * The Captain says he told his wife he'd be home for Christmas.

The Doctor

 * Due to the timelines being out-of-sync, the Second Doctor will forget about trying not to regenerate and so the Twelfth Doctor does not remember it.
 * The First Doctor's face is, according to the Twelfth Doctor, "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 Twelfth Doctor calls the First Doctor Mister Pastry, Mary Berry, and Corporal Jones.
 * The Doctor is called "The Destroyer of Worlds", "The Imp of the Pandorica", "The Oncoming Storm", "The Shadow of the Valeyard", "The Beast of Trenzalore", "The Butcher of Skull Moon", "The Last Tree of Garsennon", "The Destroyer of Skaro" and "the Doctor of War".
 * The Twelfth Doctor attempts to distract the First Doctor with the blinking lights on his astral map after his unfavourable remark to Bill about the Twelfth Doctor needing her for cleaning his TARDIS.
 * The First Doctor says he travels 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. He explains that this is only what he ran to, not what he ran from.
 * The jacket that the Third Doctor wore on his trip to Spiridon is hanging in the Twelfth Doctor's TARDIS.
 * The Twelfth Doctor's ring falls off of the Thirteenth Doctor's hand, as it doesn't fit on her finger.

TARDIS

 * The First Doctor calls the TARDIS "the Ship", much to the Twelfth Doctor's nostalgic joy.
 * The First Doctor remarks on the exterior changes between his and the Twelfth Doctor's TARDISes; notably, the windows being the wrong size, the blue colouring having changed and its size having expanded.
 * The First Doctor disapproves 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, [it's] not a restaurant for the French".
 * The Twelfth Doctor still has blackboards in his TARDIS.
 * Bill remarks the windows on the First Doctor's TARDIS exterior 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.
 * The Twelfth Doctor's TARDIS interior ends up exploding due to "Multiple Operations Failures" after the Thirteenth Doctor causes a "Systems Crisis" by clicking a red button on the console. The exploding TARDIS throws the Doctor out as it dematerialises.

Music

 * The British soldiers sing "Silent Night" and the German soldiers sing the German version, known as "Stille Nacht".

Story notes

 * Though brief, this was the debut performance for Jodie Whittaker as the Thirteenth Doctor. Her scene was written by the incoming showrunner Chris Chibnall.
 * The cinematic, DVD and Blu-ray releases 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 the 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. This scene makes use of two aspect ratios. The footage reframes from 4:3 black and white to 16:9 colour, slowly morphing Hartnell into David Bradley, and Lily Travers and Jared Garfield then appear as Polly and Ben, reenacting missing segments on Snowcap from said episode. Following the First Doctor's return to his TARDIS, the episode briefly goes back to 4:3 black and white after Bradley's Doctor is seen lying on the floor unconscious, cutting to the original regeneration from Hartnell into Troughton, and visually bookending the First Doctor's involvement in the narrative.
 * This story features a rare instance in which actors that only appeared through use of archive footage were also credited. Another notable instance of this happening is TV: The Day of the Doctor. William Hartnell's credit has been changed from "Dr. Who" to "The Doctor."
 * 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 occurred 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 fifth person to have both written for and acted in Doctor Who. The others being Victor Pemberton, Derrick Sherwin, Glyn Jones and Mark Gatiss. Though Whithouse and Gatiss are the only two who have written and acted in the revived series and are the only two to have written multiple stories. Gatiss and Whithouse were the most prolific writers during Steven Moffat's time as showrunner after Moffat himself).
 * Mark Hedges, 1st assistant director, last worked on a DWU programme on Invasion of the Bane, the very first episode of The Sarah Jane Adventures in 2007. Gareth Meirion Thomas, sound recordist, last worked on Torchwood: Miracle Day in 2011. This was the first, and only, Doctor Who credit for both crew members.
 * 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.
 * 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.
 * On the last day of filming, Mark Gatiss brought in the jacket that Jon Pertwee wore in TV: Planet of the Daleks. It was featured hanging from a stand in the background in the Twelfth Doctor's TARDIS during the regeneration scene. He also published a Behind the scenes picture of Capaldi wearing the jacket.
 * 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.
 * Jenna Coleman's appearance had to be recorded separately as she was too busy working on Victoria. She shot her scene in front of a green screen at the Top Gear office.
 * The First Doctor's assertion that "I can't go through with it. I will fight it. I will not change." as he walks through the snow at the start of the episode is a line that was originally cut from the regeneration scene at the end of The Tenth Planet, according to the production subtitles available on the The Tenth Planet DVD release.
 * For the First Doctor's TARDIS, the wall with photographs of roundels was replaced with the wall behind the Curator from TV: The Day of the Doctor.
 * When the Doctor's ring falls off her finger, only the secondary band of the ring was in the shot. This was because the first band of the ring was Peter Capaldi's wedding ring.
 * This is the first multi-Doctor story to explicitly take place during a previously broadcast TV story from the earlier Doctor's perspective, though footage from the unbroadcast serial TV: Shada was used in TV: The Five Doctors.
 * This was the last episode contributed to by production designer Michael Pickwoad before the announcement of his death in August 2018.

Ratings

 * 5.70 million (UK overnight)
 * 7.92 million (UK final)
 * 1.15 million (BBC America overnight)
 * 2.2 million (BBC America final)

Filming locations
to be added

Production errors

 * Bill's shirt changes constantly throughout the entire episode, from the rainbow shirt that was seen in Smile, to a new shirt which we have not seen before.
 * However, this could be attributed to the fact that this version of Bill is a collection of her memories and not her physical self.
 * Nikki Amuka-Bird fluffs a line: “Step out of the caps-ul.” She should have said “Cap-sule.”
 * When examining the glass woman, the Twelfth Doctor’s right hand is in line with his waist. Next shot and his hand is down his side. In the next shot, his left hand is next to his waist.
 * When the Captain is taken from the World War One battlefield he has his revolver in his hand. After he falls to the ground at the South Pole, the gun is in his holster and he has to draw it again.
 * At around the 39-minute mark, the date on the electronic display reads 5,000,000,012 & the Doctor says year five million & twelve. 5,000,000,012 is five billion & twelve.
 * A few times throughout the episode, the doors of the first Doctor's TARDIS open as he approaches them even though he didn't operate the door control on the console.
 * After the Captain requests a favour from the Doctors, the First Doctor can be heard saying "Oh, anything. Name it," but his mouth is not actually moving after the word "oh."
 * In the Christmas scene on the battlefield, fire is seen in several places. In the shots where objects are in front of the fire, the smoke is seen to rise in front of the object, despite the fire being behind.
 * When the medics run with the stretcher, it is brown and has a large stain on it. But when the soldier is on it, it changes to black.
 * During the scene in which the soldiers are playing football, the shot is flipped. This is visible due to the soldier's uniforms.
 * The Twelfth Doctor begins regeneration with his back to the stairs, but immediately following regeneration, the Thirteenth Doctor is standing at a different angle and is further away from the stairs.
 * When the Doctor catches herself from falling through the TARDIS doors with her foot, her boot briefly changes to a different shoe.
 * In the shot where the Doctor falls through the TARDIS doors, the bottom-right section of wall has clearly been digitally extended on the last frame.

Myths

 * The Twelfth Doctor would be killed. The Twelfth Doctor was killed in the previous story, he just refused to regenerate then.
 * Because the opening was set in the events of The Tenth Planet, Cybermen would appear. Although this was technically true, they only appeared through the use of archival footage from said story.
 * A full recreation of several scenes were originally filmed for this purpose.
 * Carole Ann Ford would return as Susan Foreman. This was proven false.
 * The Captain would turn out to be Gilbert MacKenzie-Trench, the man who created the iconic police box design. The Captain was revealed to be Archibald Hamish Lethbridge-Stewart, an ancestor of the Brigadier.
 * The Doctor would regenerate while falling. The Doctor fell out of the TARDIS after regenerating instead.
 * According to the Daily Mirror, this story would see the last day of the Time War from the perspective of the First Doctor and the Twelfth Doctor. This was proven false.
 * Matt Smith would return as the Eleventh Doctor. This was proven false, although archive footage was used of all of the Doctor's incarnations up to the Twelfth by the Testimony to prove to the First Doctor that the Twelfth Doctor was indeed his future incarnation.

Continuity

 * The Twelfth Doctor mentions Bill's Cyber-conversion, (TV: World Enough and Time) and Bill explains how Heather saved her, though she has difficulty remembering the details. (TV: The Doctor Falls) Bill has to remind the Twelfth Doctor about Heather and the sentient oil. (TV: The Pilot)
 * When time freezes following the meeting of the Twelfth and First Doctors, the Twelfth theorises that it is their temporally-paradoxical meeting which has caused this effect — much as when time was similarly frozen when the Eighth Doctor encountered the First Doctor. (PROSE: The Eight Doctors)
 * 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 nearly encountered his first incarnation when the TARDIS was accidentally transported to the barn on Gallifrey where his first incarnation was hiding as a child. However, the Twelfth Doctor was unconscious at the time and left without going outside to meet his younger self. (TV: Listen)
 * 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. (HOMEVID: Last Night)
 * 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 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)
 * Villengard was once called the nightmare of the Seven Galaxies, a Kaled concept that believed only Skaro could have intelligent life forms in the seven galaxies. (TV: Genesis of the Daleks)
 * Villengard is at the centre of the universe, where Terminus was when the Fifth Doctor visited the area. (TV: Terminus)
 * 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) mentioning many "pressing reasons" as the Fourth Doctor did to Adric. (TV: Logopolis)
 * The Twelfth Doctor meets Rusty again. Rusty calls the Twelfth Doctor a "good Dalek", as he did before. (TV: Into the Dalek)
 * The Doctor mentions that Earth is a "level 5 civilisation". (TV: City of Death, Voyage of the Damned, The Eleventh Hour)
 * As Bill, the Doctor and Nardole hug, the latter says "cuddle". He previously said this when hugging the Doctor and Bill. (TV: Oxygen)
 * 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)
 * The First Doctor remarks on how dusty and dirty the Twelfth Doctor's TARDIS is. Wilfred Mott once said similar regarding the Tenth Doctor's TARDIS, specifically that he felt it should be cleaner. (TV: The End of Time)
 * The Twelfth Doctor repeats River Song's favourite phrase of "spoilers" to the Captain and his predecessor. The Tenth and Eleventh Doctors also employed this phrase. (TV: Silence in the Library, Let's Kill Hitler, The Day of the Doctor) 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 databank. (TV: Castrovalva)
 * The Twelfth Doctor mentions the Daleks' Pathweb. (TV: Asylum of the Daleks)
 * The Twelfth Doctor once more references River's words from her diary. (TV: Extremis, The Doctor Falls)
 * 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 and eyebrows, 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 First Doctor chides Bill for swearing, echoing how he did the same 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 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)
 * 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 at the pleasure of the human race". (TV: Thin Ice)
 * The First Doctor threatens Bill with a "jolly good smacked bottom". He told Susan Foreman she also deserved "a jolly good smacked bottom" after her clumsiness inconvenienced him. (TV: The Dalek Invasion of Earth)
 * 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)
 * The Testimony storing the memories of the dead is similar to the APC Net, (TV: The Deadly Assassin) and the Nethersphere. (TV: Dark Water)
 * Bill knows that the Doctor has had different faces. (COMIC: The Lost Dimension)
 * The Doctor regains his memories of Clara Oswald after seeing her glass avatar. (TV: Hell Bent)
 * "Clara" is wearing the clothes she died in. (TV: Face the Raven)

DVD releases
A stand-alone DVD was released on January 22nd, 2018.

Blu-ray releases
A stand-alone DVD was released on January 22nd, 2018 with special features including a recording of the Doctor Who panel at San Diego Comic-Con 2017.

Digital releases
to be added