The Doctor Falls (TV story)

 was the twelfth and final episode of the tenth series of Doctor Who produced by BBC Wales. It saw the departures of both Bill Potts and Nardole, and the apparent end of the Master's life.

Expanding on the Twelfth Doctor's regeneration teased in the previous episode, it saw him mortally wounded in battle against the Cybermen and two times his hand glows, but he manages to hold back. The regeneration would also start up fully two times, both of which he managed to postpone, however, as the Doctor refused to change, displaying how he hates having to be different people when he grows accustomed to being in an incarnation for a long period of time, mirroring the Tenth Doctor's attitude towards the process.

The episode also shows the events leading to 's regeneration into, with the male Master being mortally wounded by his female successor, and he in turn seemingly killing her, this being an example of why, unlike the Doctor who has teamed up with his past and future incarnations, the Master very rarely works with any of his past or future incarnations as he would eventually double-cross himself. With the mystery of how the Master had regenerated into Missy now being shown, this would mark the last appearance of John Simm as the "Harold Saxon" incarnation of the Master (at least in his natural stream of events).

The story featured several connections to the 1966 serial The Tenth Planet, including a story featuring humans from Mondas, and featured appearances by the Cybermen from said planet.

The episode featured the unexpected return of the First Doctor, now portrayed by David Bradley (who had previously portrayed William Hartnell in the docudrama An Adventure in Space and Time). This marks the first time since The Five Doctors that the adult First Doctor had been portrayed outside of archive footage, after the First Doctor appeared as a child in Listen, and the second time since The Tenth Planet where the First Doctor was in a regular episode of the show, even though it was a short appearance.

Synopsis
As he struggles to resist regenerating, the Twelfth Doctor prepares to make a last stand against the ever growing army of Cybermen. However, with Bill still a Cyberman, and gradually turning to her old ways as  reminds of her of who she used to be, can the Doctor convince his old friend to finally be good before he falls?

Plot
The episode opens to a group of farmers, adults and children, on Floor 0507 facing against the prototypes from Floor 1056, dubbed "Cyber-scarecrows", shooting them down by night to then restrain on wooden crosses by day. A school child is out in the field one morning when a shuttlepod blasts through the ground, crash landing near her. From the rubble, a Cyberman walks out carrying an unconscious Doctor.

Sometime earlier, the Doctor awakes on top of the hospital, soon recalling how he was subdued by and  and Nardole fled, He deduces that the Master was exiled from Gallifrey and has been hiding on the colony ship. The Doctor, now restrained in a wheelchair, watches the two incarnations of the Master taunt him on the many deaths the Doctor has had and wanting to find out new ways to kill him. The Master and Missy dance and flirt with each other and gloat of their plan of the Cybermen converting all the humans on the ship. They mockingly call the Doctor "granddad", reminding him that Bill is now a Cyberman.



However the Master is stunned to see that the Cybermen are heading towards the building that they are perched on. The Doctor explains that when he was knocked down by Missy, he altered the signal of the Cybermen so they would convert two-hearted beings as well, hence why they are now focusing on the Time Lord's presence. The Masters panic, Missy frantically using her sonic umbrella to lock a metal door to hold back advancing Cybermen while the Master uses his laser screwdriver to destroy advancing Cybermen. The Doctor brags that he is the only one who is has able to stop so many Cybermen at once. Realising this, a conflicted Missy knocks the Master out and frees the Doctor from his restraints. The Doctor starts wondering which side she is on to which she admits her indecisiveness as she is confused and her other half is unconscious.

The Doctor uses his sonic screwdriver to contact Nardole, who reveals he has commandeered a shuttlecraft, lands near the roof to help escape, but the Doctor is grabbed and electrified by a Cyberman before he boards. Missy attempts to save him using her other self's laser screwdriver but Bill, in her cyber form, intervenes first, destroying the Cyberman with a blaster based on her head. Missy and the Master try to convince Nardole to abandon the Doctor, but Bill prevents the shuttle from leaving, grabbing the ladders until they rescue him.

They escape to a higher floor on the ship, where time moves slower. They find themselves on a solar farm, the Master due ducking such by the holographic sky, where some of the humans and a large number of children protect themselves from the early Cybermen prototypes from Operation Exodus. Nardole approaches the girl, declaring they will need her help. They spend two weeks there while the Doctor recovers from his injuries. Bill awakes, in the barn, unsure why the villagers are scared on her, concerned why she can't see the Doctor. When given a mirror by a child, she becomes horrified to discover she was converted into a Cyberman. The girl goes to run but bumps into the Doctor who thanks her for being kind to Bill, offering her a jelly baby as she leaves. The Doctor has somewhat recovered from his injuries, though he still has a scar from where he was hit by the Master.

Trying to console Bill, she gradually realises that though she thinks she is human, she is still a Mondasian Cyberman in appearance. She grows angry, the Doctor backing away as Bill accidentally triggers the blaster on her head, igniting the door on the barn, causing an explosion. Nardole, who has been preparing the town for the oncoming Cybermen despite one women saying he has been working them too hard, passively tells the farmers not to worry; they have an upcoming war to focus on.

Bill sheds a tear when she realises the humans are frightened of her, and the Doctor wipes it away, noting it unusual. The Master tells the Doctor that he and Missy have been working on something, and pauses briefly to mock Bill, chastising the Doctor for still referring to her as a female and taunts how he hated the time he had to spend with Bill as "Razor". He stops because Bill claims he is not upsetting her, though she cries beneath her mask when he leaves. The Doctor finds it odd she is still able to shed tears. While traversing after the Master, the Doctor struggles to walk, limping heavily. He briefly stumbles against a tree, regeneration energy attempting to trickle from his hand. Wanting to not worry Bill, he draws up a stick and carries onwards but Bill is still distraught that the Doctor can't fix her current condition, the Doctor remarking there is still hope as she is strangely able to cry.



They soon reach the middle of a forest, just after the Master and Missy theorise they can't retain their memories as their timelines are out of sync. Missy briefly displays empathy for Bill's condition, something the Master finds disgusting. Missy reveals she has discovered a camouflaged lift in the nearby forest, which she suggests they can escape to the bridge, using her sonic umbrella to call it. However, the Doctor declares that there will no doubt be a Cyberman in the lift waiting to find their floor, as the lift was taken to the bottom by them previously. Understanding this, Bill orders them to stand aside, the Doctor convincing the two Masters to do so. The three Time Lords prepare behind Bill, drawing their respective devices. As guessed, the lift brings up an advanced Cyberman from the lower floor, Bill speedily firing on it with her cyber blaster, the Master following likewise with his laser screwdriver as the Doctor and Missy use their sonic devices to slow its advance with a barrier. After several seconds, they disable it.

The Doctor warns that (through the Cybernet) they will now have much more time to plan, given they now know what floor they reside on, so they must prepare for a battle. The Doctor returns to the village, revealing the alarm they now hear is the Cyberman's warning from the lower floors. Using a computer, Nardole discovers that the floor below this one has many fuel pipes that he can detonate as weaponry against the Cybermen, briefly testing such by blowing up a wind mill, stunning the villagers; the Doctor remarks that they can use such to confuse the Cybermen and make them believe they are stronger than them. The Doctor finds a conduit to another solar farm on a higher floor that can be used to evacuate the children, having Bill blast it open, knowing this is a fight they will likely lose.

Meanwhile, the Master and Missy decide to leave and plan to find the Master's TARDIS at the lowest floor, though the Master reveals he broke his TARDIS. Missy convinces her younger self to always keep a dematerialisation circuit on hand, revealing she now does. Night falls, the Doctor standing guard outside the house. Inside, a woman attempts to flirt with Nardole as he works on his computer, only to turn around and shoot at Bill, falsely thinking her a hostile Cyberman. She leaves distraught, going outside to the Doctor. The first wave arrives, Nardole and a girl preparing to combat them, successfully tricking them that an apple was capable of destroy an entire group of them. They retreat to the house, now understanding the Cybermen will likely return to outright destroy them next time rather than merely capture them for conversion.



Meanwhile, the Doctor notices the two Masters trying to leave and runs after them. He pleads for them to be by his side to help, as saving people would just be kind. The Master ridicules the idea, saying that he didn't listen to a word the Doctor said. However Missy was somewhat moved by the Doctor's speech, agreeing being the Doctor's friend was what she always wanted. However, after briefly grabbing each other's hand, she sides with her predecessor. At the lift, Missy tricks the Master into dancing with her and fatally stabs him, forcing his next regeneration, though she makes the wound "precise" so that the Master will have enough time to reach his TARDIS before regenerating. Gradually dying, the Master compliments her, glad they haven't lost their touch. After helping him into the elevator, Missy announces she will return to help the Doctor, turning to walk away as her predecessor shouts out for her to not go. Unable to accept that his future self will side with the Doctor, the Master draws and fires his laser screwdriver at full blast, causing Missy to collapse on the ground, fatally wounded. He claims the strike has disabled her regeneration, commenting that it is the perfect ending, with them both shooting themselves in the back. They both laugh as the Master descends in the lift and Missy seemingly dies.

An initial wave of Cybermen arrive, but using Nardole's tricks with the explosives, they are able to make the humans appear more powerful, forcing the Cybermen to retreat and develop a new plan, giving the humans more time. Back at the house, and against Nardole's protests, the Doctor downloads the plans for the floor into his sonic screwdriver and sends him to help evacuate the children, intending to stay alone to fight the Cybermen. Nardole resists, having deduced the Doctor is going to commit suicide by detonating the entire floor in a last ditch attempt to destroy as many Cybermen as possible. The pair argue over who really should die, Nardole reasoning he can't given how he was before meeting the Doctor even though the Doctor tries to reason that he should stay as he is already dying. The Doctor eventually convinces Nardole that helping the children will be his penance for the crimes he committed before they met, as he is the stronger of the pair. Nardole jokes he will name a farm after him. Bill also insists on staying with the Doctor despite Nardole trying to tell her otherwise. As he leaves with the humans, Nardole admits that he will never be able to find the words for Bill and the Doctor's sacrifice.



With all the humans evacuated, the Doctor and Bill prepare for a last stand, each deciding to charge into opposite ends of the forest. Bill says one last goodbye to the Doctor, even joking briefly about her sexuality, assuring him she had still retained her humanity. They face the oncoming wave. The Doctor ignites several of the Cybermen, listing the numerous times he has defeated them while doing so. However, he is inevitably surrounded and receives a full strike from one Cyberman's laser beam. He declares he is the Doctor but the Cybermen retort they have no need for a doctor anymore, firing at him again. Falling to his knees, he prepares to ignite the floor but it struck again, the blast proving fatal enough that his body attempts to regenerate. Refusing to give in, he holds it back and ignites all the pipes below the floor, engulfing it all in a wave of destruction, destroying all the Cybermen.

Above on floor 0502, Nardole waits outside of one of the lifts, hoping the Doctor and Bill will return however, the children convince him to move on and focus on living with them now.

Back downstairs, the floor is now a wasteland scattered with destroyed Cybermen. Bill, barely functioning, finds the Doctor severely wounded. She mourns over the Doctor's body, but suddenly finds herself out of the Cyberman suit, seemingly human. She looks up and sees Heather (from The Pilot) who found her by her tear and changed her into a being like Heather. They take the Doctor's body to the TARDIS, and Heather, as the Pilot, directs the TARDIS to a new location, while offering Bill to live out a different life, and Bill accepts. Before leaving, Bill sheds a tear on the Doctor, but does not wipe it away, saying "Where there's tears, there's hope."



The TARDIS lands in an icy planet. The Doctor, after dreaming of some of his previous companions, wakes up and starts regenerating. He tries fighting it off, recalling the last words of some of his previous incarnations in his confusion. He manages to control the regeneration, angrily stating that he does not want to change again, hating the fact that he must become someone else. He steps outside, declaring to the TARDIS it doesn't matter where she has taken him, he won't listen. Out in the snow, yet again his regeneration attempts to start. The Doctor screams no and stops the regeneration by shoving his fists into the snow, again stating he will not change. Suddenly in the distance he hears a voice stating the same words of not changing, finding the idea ridiculous. The Doctor calls out to the person. The stranger asks for the Doctor's identity, which he confirms. The stranger states that while the Doctor may be a Doctor, he "is the Doctor, the original you might say". Much to the Doctor's shock and surprise the stranger is revealed to be his first incarnation.

Cast

 * The Doctor - Peter Capaldi
 * Bill - Pearl Mackie
 * Nardole - Matt Lucas
 * - Michelle Gomez
 * - John Simm
 * Hazran - Samantha Spiro
 * Alit - Briana Shann
 * Gazron - Rosie Boore
 * Rexhill - Simon Coombs
 * Heather - Stephanie Hyam
 * Voice of the Cybermen - Nicholas Briggs
 * and


 * The Doctor - David Bradley

Cybermen

 * The expedition to Floor 0507 appear to be used as scarecrows.

Vehicles

 * Nardole pilots a shuttle labelled SL-16.

Music

 * The song heard when Missy and the Master are dancing on the rooftop is "Midnight, the Stars and You" by Al Bowlly.

Story notes

 * This marks David Bradley's first appearance as the First Doctor in a DWU television story. Bradley previously portrayed a fictionalised version of William Hartnell, the original actor to portray the First Doctor, in the non-DWU docudrama An Adventure in Space and Time, in which Bradley reenacted scenes from several First Doctor serials. Bradley becomes the fourth actor to play the First Doctor in a televised story following original actor Hartnell, Richard Hurndall (TV: The Five Doctors) and the uncredited child actor who played him in TV: Listen. He also becomes the fourth actor, following Colin Baker, David Tennant and Peter Capaldi, to play the Doctor after having previously played another character(s) in the franchise (in Bradley's case, Solomon in TV: Dinosaurs on a Spaceship).
 * The Doctor's mention of Marinus as a planet on which the Cybermen originated (COMIC: The World Shapers) is another rare instance of the TV series directly referencing non-televised material.
 * This episode literally removes any contradiction that had previously revolved around the various Cybermen origin stories, by having the Doctor call all instances parallel evolution.
 * This episode wraps up the stories of several recurring characters;
 * Bill and Heather begin their own travels through time and space, Bill leaving the Doctor behind in the TARDIS.
 * Nardole stays behind on the Mondasian ship to help Hazran and her children escape the Cybermen. (His ultimate fate, however, is not revealed.)
 * is forced to regenerate by Missy after she stabs him. Both agree that he then regenerates into Missy, in his own TARDIS, and he forgets the whole adventure. However, his actual change is not shown on screen.
 * is killed by her previous incarnation, left unable to regenerate. In their own words, they were shot in the back by themselves.
 * On original broadcast on HD, a technical fault led to sound problems which rendered the dialogue inaudible for the final five minutes: from Bill leaving the TARDIS to the end credits.
 * The use of the number 507 is a nod to Russell T Davies, who often invokes the number 57 in his writings, and who once had the Eleventh Doctor claim to Clyde Langer that he could regenerate 507 times (TV: Death of the Doctor - the only Eleventh Doctor story written by Davies).
 * A flashback sequence features the Doctor remembering virtually all of his 2005-present companions, including Jack Harkness and Sarah Jane Smith, both of whom appear on screen for the first time in the Steven Moffat era (projected images of Martha Jones, Donna Noble and Rose Tyler had appeared in TV: Let's Kill Hitler, with The Moment taking Rose's form in The Day of the Doctor).

Ratings

 * 3.75m (UK overnight figures)

Filming locations

 * Caerwent, Wales - Floor 0507 farmland

Production errors

 * After Bill blows up the barn, a cameraman is visible on the edge of the shot.
 * In the forest scenes, several pieces of camera equipment are somewhat visible, such as a dolly and a tripod.

Continuity

 * Bill carries the Doctor's unconscious body in much the same manner that the Eleventh Doctor and Rory Williams once carried Amy Pond. (TV: Asylum of the Daleks, The Girl Who Waited) This isn't the first time the Twelfth Doctor has been physically carried in such a manner by a companion, as Clara Oswald also previously carried him this way, though he was not unconscious. (COMIC: Clara Oswald and the School of Death)
 * The expedition to 507 that "Razor" talked about are being used as scarecrows. (TV: World Enough and Time) They, however, walk free at night and attempt to attack the collage, but are shot down with rifles. (TV: Human Nature/The Family of Blood)
 * The clock on the hospital is seen ticking towards one o'clock (or thirteen), away from twelve, shortly before the Doctor is severely injured. Clocks have previously been seen as an indication of the Doctor's impending regeneration. (TV: The Time of the Doctor)
 * The Doctor recalls last seeing Missy's predecessor heading to Gallifrey, deducing that the Time Lords cured his "little condition" before kicking him out. (TV: The End of Time)
 * The Master gloats over his dominion to a wheelchair bound Doctor just as he did with the aged Tenth Doctor during the Year That Never Was, even calling him "Granddad", (TV: Last of the Time Lords) and to a restrained Doctor upon the rise of the Master Race. (TV: The End of Time)
 * The Doctor notes the Cybermen have risen naturally from Mondas (AUDIO: Spare Parts, TV: The Tenth Planet) Telos, (PROSE: Doctor Who and the Cybermen) Earth, (TV: Rise of the Cybermen / The Age of Steel) Planet 14 (TV: The Invasion) and Marinus. (COMIC: The World Shapers)
 * The Doctor recounts his various victories over the Cybermen: on Telos where he sealed them into their ice tombs, (TV: The Tomb of the Cybermen) Voga, (TV: Revenge of the Cybermen), Canary Wharf (TV: Army of Ghosts / Doomsday) and Planet 14. (COMIC: The World Shapers, TV: The Invasion) He states: "Every single time, you lose, even on the Moon". (TV: The Moonbase).
 * The Doctor reminds Bill how she remembered the truth during the reign of the Monks. (TV: The Lie of the Land)
 * The Doctor reiterates his dislike of guns, (TV: The Sontaran Stratagem, The Doctor's Daughter, The Impossible Astronaut, The Curse of the Black Spot) despite having used them several times. (TV: Day of the Daleks, The Invasion of Time, Hell Bent)
 * Nardole reminds Bill that he is still allowed to "kick [her] arse". (TV: Extremis) Bill tells him he will have to go down to the hospital to find it then. (TV: World Enough and Time)
 * Nardole mentions how he used to be a black market dealer before meeting the Doctor, (TV: The Husbands of River Song) and further indicates he is a cyborg. (TV: Oxygen, The Eaters of Light, World Enough and Time)
 * Nardole states that Hazran is "only human", something the Doctor has said numerous times. (TV: The Big Bang, A Good Man Goes to War)
 * Missy is able to provide the Master with a dematerialisation circuit by telling him to always carry a spare, similar to how the Tenth Doctor told the Fifth Doctor how he could stop the TARDIS exploding. (TV: Time Crash)
 * Missy telling her previous incarnation she loved being him also mirrors a sentiment of the Tenth Doctor, as he expressed such to his fifth incarnation. (TV: Time Crash)
 * The Doctor reiterates River's words that Nardole once spoke to him from her diary. (TV: Extremis)
 * Bill is reunited with Heather in the form of sentient oil. (TV: The Pilot)
 * Bill telling the two Masters to stand aside so she can fire at the upcoming Cyberman echoes when the Tenth Doctor told the Master to do likewise when he had a revolver and vice versa as the Master prepared to attack Rassilon. (TV: The End of Time)
 * Missy uses her sonic umbrella. (TV: World Enough and Time)
 * The Master asks the Doctor if he has ever burned to death. Although the Doctor hasn't, the Master has. (TV: Planet of Fire) Missy says that she knows the Doctor has literally fallen to his death as the Fourth Doctor regenerated as a result of falling off the Pharos Project's radio telescope. (TV: Logopolis) The Master asks if the Doctor has ever drowned, which he did while fighting the Racnoss in an alternate timeline. (TV: Turn Left) Missy asks if the Doctor has ever felt the blade, with something similar actually happening to him. (TV: Doctor Who)
 * The Master uses a modified laser screwdriver. (TV: The Sound of Drums/Last of the Time Lords)
 * The Master reveals to Bill that he was Mr. Razor and that he hated the 10 years they spent together on floor 1056. (TV: World Enough and Time)
 * The Master (seemingly) once again refuses to listen to the Doctor. (TV: Last of the Time Lords) However, Missy does, in the end, turn back to join him, as she has progressively had a change of heart. (TV: Extremis et al.)
 * Bill chooses the TARDIS as the Doctor's final resting place. In an alternate timeline, the Eleventh Doctor was interred in the TARDIS after his death on Trenzalore. (TV: The Name of the Doctor)
 * The Doctor has a brief recall of some of his past companions from his ninth incarnation onwards, just before his body attempts to regenerate. The Fourth and Fifth Doctor similarly had this occur to him. Likewise, the last face he sees is the Master's. (TV: The Caves of Androzani, Logopolis)
 * He recalls Rose Tyler from TV: Bad Wolf, Martha Jones from TV: Daleks in Manhattan, Donna Noble from TV: Partners in Crime, Jack Harkness from Utopia, Vastra from TV: The Name of the Doctor, Jenny Flint from TV: The Crimson Horror, Sarah Jane Smith from TV: The Stolen Earth, Amy Pond from TV: The Impossible Astronaut, River Song from TV: The Pandorica Opens, and even Clara Oswald from TV: Last Christmas, despite having her wiped from his memories. (TV: Hell Bent) He also recalls Missy from TV: The Witch's Familiar)
 * When waking up after his pre-regeneration flashbacks, the Doctor says "Sontarans! Perverting the course of human history!". They were the first words of the Fourth Doctor after regenerating. (TV: Robot). He previously said the same thing after waking up from being knocked out by flying objects due to air pressure. (TV: Listen)
 * As he begins regenerating, the Doctor says "I don't want to go", the exact same words his tenth incarnation said before regenerating. (TV: The End of Time) He also reiterates the last lines of his eleventh incarnation; "when the Doctor was me"; (TV: The Time of the Doctor) as well saying "it's starting. I'm regenerating", the final words the Tenth Doctor stated before his aborted regeneration. (TV: The Stolen Earth) He also shouts "no!" repeatedly while his regenerative energy flairs up, which were the final words of the Second Doctor (TV: The War Games - before expanded media showed the true circumstances of his regeneration in COMIC: The Night Walkers)
 * The Doctor resists regenerating. He has previously done this when nearing the end of his fifth (TV: The Caves of Androzani) and tenth incarnations. (TV: The End of Time)
 * The Doctor offers people jelly babies. (TV: The Three Doctors, Robot, et al.)
 * The Master refers to the Doctor as "Doc". The Doctor's first incarnation had previously taken umbrage at being addressed like that (TV: The Time Meddler, The Five Doctors), as did his sixth (TV: The Twin Dilemma, The Ultimate Foe) and tenth. (TV: Dreamland)
 * It was revealed Heather gave her tears to track Bill, explaining how Bill was still able to cry despite being a Cyberman. (TV: World Enough and Time)
 * The cyber-converted Yvonne Hartman, who had managed to retain her free will after conversion, also displayed the ability to cry; however, she cried oil rather than actual tears like Bill. (TV: Doomsday)
 * The Doctor meets his first incarnation. He has met this version of himself on numerous previous occasions. (TV: The Three Doctors, The Five Doctors, The Day of the Doctor)
 * The Doctor and Bill both recite the line "while there's tears, there's hope" at different points in the story. A similar line, "while there's life, there's hope," served as the final words of the Third Doctor, while trying to console a crying Sarah Jane Smith. (TV: Planet of the Spiders)
 * As the Doctor leaves the TARDIS the cloister bell is heard ringing. (TV: Logopolis et al.)
 * The Twelfth Doctor, and later the First Doctor, describe themselves as "the Doctor, the original you might say." The First Doctor previously said this exact phrase to Tegan Jovanka. (TV: The Five Doctors).
 * Missy severely injures one of her previous incarnations. The Master previously did it to himself. (AUDIO: The Two Masters)
 * Bill was strong enough to survive the cyber-conversion process while retaining her humanity, largely in part due to the strong attachment she had to her deceased mother. (TV: The Pilot et al.)
 * Yvonne Hartman and Mercy Hartigan were also able to retain their consciousness following their conversions. (TV: Doomsday, The Next Doctor)
 * The Doctor had previously encountered a version of Clara Oswald who had been strong enough to survive a Dalek conversion process and retain her humanity, she too had few memories of the conversion and believed she was still human. (TV: Asylum of the Daleks)
 * Craig Owens had previously been able to resist a Cyber-conversion due to his strong love for his son overpowering a damaged cyber-ship's conversion equipment. (TV: Closing Time)
 * The Master being stranded by a malfunction, or by his TARDIS otherwise being unavailable to him has happened to him several times. (TV: The Claws of Axos et al., AUDIO: Sympathy for the Devil et al.)
 * The Doctor comments that it's a pity there are no stars in view when he seemingly dies, echoing an affection voiced earlier in his life: "the deep, lovely dark; you'd never see the stars without it." (TV: Listen)
 * Missy carries a spare dematerialisation circuit because she remembers telling her previous incarnation to always carry one, in much the same way the Tenth Doctor once remembered how to get out of a predicament because his fifth incarnation saw how he did it and remembered. (TV: Time Crash)
 * The Masters discuss how an earlier incarnation cannot clearly remember what they learn from meeting a future one. (TV: The Day of the Doctor)

DVD releases
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