Transitions
The Fourth Doctor regenerates into the Fifth Doctor (from Logopolis, 1981 andCastrovalva, 1982) The regeneration "effect" was accomplished during the series' original run from 1963–1989 primarily through the use of video mixing. Originally, the plan was to have Hartnell collapse at the end of The Tenth Planet with his cloak over his face, which would then be pulled back to reveal Troughton in the next serial. However, vision mixer Shirley Coward discovered and took advantage of a malfunction in the mixing desk which allowed Hartnell's image to be overexposed to the point of almost whiting out the screen, then fading back in to reveal Troughton's face. This also meant that the regeneration scene could take place with both actors at the conclusion of The Tenth Planet, and Troughton was accordingly signed up to participate.[1]
Subsequent regenerations retained essentially the same method, with or without additional video or make-up effects. The transition from the Fourth to the Fifth Doctor used an additional make-up effect representing a transitional form known as the Watcher, but aside from this, other regenerations in the original series run simply mixed the image of the incoming actor on top of the outgoing one. The transition from the Seventh to the Eighth Doctor in the 1996 television movie took advantage of the higher budget and modern computer animation technology to "morph" the features of Sylvester McCoy into those of Paul McGann.
With the exception of the transitions from the Second to Third and the Eighth to Ninth Doctors, each regeneration was shown on-screen, with the previous incumbent in the role symbolically "handing off" the character to the next. The Second Doctor was never seen to actually change into the Third, simply fading off into darkness at the end ofThe War Games and then stumbling out of the TARDIS, already regenerated, at the start of Spearhead from Space (1970).
The regeneration of the Sixth Doctor into the Seventh is the only time that a single actor took on the roles of two incarnations of the Doctor. Colin Baker declined the invitation to film the regeneration sequence at the start of Time and the Rani (1987) due to the circumstances in which the BBC dismissed him from the role.[5] As a result, Sylvester McCoy had to don his predecessor's costume and a blond curly wig, lying face down, with the mixing effect to the Doctor's "new" features occurring as he was turned over.
The 2005 series, which revived the programme after a 16-year hiatus, began with the Ninth Doctor already regenerated, with no explanation given as to the circumstances behind the change (although dialogue within the debut episode, Rose indicated the change had recently occurred). In the documentary series Doctor Who Confidential, producer Russell T Davies explained his reasoning that, after such a long hiatus, a regeneration in the first episode would not just be confusing for new viewers but also lack dramatic impact, as there would be no emotional investment in the character before he was replaced.[6] The regeneration of the Ninth Doctor into the Tenth at the end of "The Parting of the Ways" (2005) was seen, and also used computer effects to morphChristopher Eccleston into David Tennant. In the episode of Doctor Who Confidential accompanying the episode "Utopia", it was stated that the production team had decided that this would be a common effect for all Time Lord regenerations (the Master's in this case) rather than having a regeneration sequence chosen on a whim by the director. A similar sequence was used for David Tennant's regeneration into Matt Smith in "The End of Time", though this time the effect on the TARDIS is much more violent, possibly due to the Doctor's attempts to fight the regeneration or the severity of the radiation poisoning that brought about the process to begin with. This regeneration effect was also used in "The Impossible Astronaut" when the Eleventh Doctor is shot twice, albeit with a more wispy effect. However in this case the full transition is not seen as the Doctor is shot again fatally before completing the process. It was later revealed that this was actually a robot impersonating the Doctor. The effect was used again at the end of the following episode "Day of the Moon" when a mysterious girl also regenerated (although her new incarnation is not depicted), and in "Let's Kill Hitler" when Mels (Nina Toussaint-White) is shot and regenerates into River Song (Alex Kingston). The same effect appears when Donna Noble (Catherine Tate) and the Tenth Doctor's spare hand together spawn a Doctor-Donna hybrid in "Journey's End".
In the series The exact mechanism that makes regeneration possible is not stated in the television series, but it is generally assumed in the spin-off media that the ability to regenerate may be linked to what is known as the "Rassilon Imprimatur", the symbiotic nuclei of a Time Lord that bonds him or her to a TARDIS, and allows his or her body to withstand the molecular stresses of time travel (The Two Doctors, 1985).[7] In "The Christmas Invasion" (2005) it was stated the regenerative cycle generates a large amount of energy that suffuses the Time Lord's body. As demonstrated by theTenth Doctor, in the first fifteen hours of regeneration this energy is enough to even rapidly regrow a severed hand.[8] In the moments following his regeneration into his eighth incarnation, the Doctor possessed enough physical strength to batter a steel door completely off its hinges. In Let's Kill Hitler River Song is able to repel bullets after regenerating.
It is first stated in The Deadly Assassin (1976) that a Time Lord can regenerate twelve times before dying (thirteen incarnations in all). There are exceptions to this rule, however; when a renegade Time Lord known as the Master finds himself at the end of his regenerative cycle, he takes possession of the body of another person to continue living (The Keeper of Traken, 1981), although he was using the Source of Traken to bind his mind to the body. It may be that the Time Lords also have the ability to circumvent the limit – in The Five Doctors (1983) the Master is offered a new cycle of regenerations by the High Council in exchange for his help. The fact that the Master is inhabiting a non-Gallifreyan body at the time implies that it is possible to grant them to a non-Gallifreyan, albeit one inhabited by a Time Lord mind (although River Song is shown to have the ability to regenerate due to altered DNA that has similarities to Time Lord DNA, a side effect of being conceived in a TARDIS). In "The Sound of Drums" (2007) the Master is revealed to have been granted a new body by the Time Lords during the Time War with at least two new regenerations. Non-Gallifreyans are also seen to regenerate in Underworld (1978) and Mawdryn Undead (1983), but with adverse side effects. In Mawdryn Undead these appear to be the result of mishandling stolen technology, but in Underworld they are implied to be the inevitable result of limited technology that reinvigorates, rather than transforms, the subject's appearance (in this case, the Minyans, with whom the Time Lords shared much of their technology), thereby regenerating 'the body, not the soul'.
The BBC's Series 4 FAQ[9] suggests that now the Time Lord social order has been destroyed, the Doctor may be able to circumvent the limit on regenerations; it says: "Now that his people are gone, who knows? Time Lords used to have 13 lives." In Death of the Doctor (a 2010 The Sarah Jane Adventures serial), the Eleventh Doctor responds to a question from Clyde Langer by saying he can regenerate "507" times. Early news reports, before the episode was broadcast, suggested he would say there is no limit to the number of regenerations.[10] Writer Russell T Davies explained in an interview with SFX that the line was not intended to be taken seriously and is instead a commentary. He insisted that the "thirteen lives" rule was too deeply entrenched in the viewer consciousness for his throwaway line to affect it.[11]
In the Fourth Doctor story The Brain of Morbius, the Doctor participates in a mental ‘duel’ with another Time Lord. The machine to which their minds are connected begins to project the faces of the "losing" contestant’s regenerations in chronologically descending order. As the Doctor is overpowered by Morbius, the images change successively to those of the third, second and first Doctors, then eight further faces appear. The narrative does not definitively assert that these are past incarnations of the Doctor rather than of Morbius; other evidence from the series suggests they cannot be. The Doctor himself has numbered his regenerations on several occasions, each time indicating that the William Hartnell incarnation was the first. For example, it is explicitly stated by the Fifth Doctor in Mawdryn Undead that he has eight incarnations left, and in The Five Doctors Peter Davison's Doctor (introducing himself to the First Doctor) says that he is the fourth regeneration, meaning that there have been five of him. Again in The Five Doctors, the First Doctor refers to himself as the "original" and in Doctor Who, Paul McGann, with Sylvester McCoy on screen, states both "A Time Lord has thirteen lives and The Master'd used all of his." and "In all my travels through space and time, and nearing the end of my seventh life...". Most recently, inThe Lodger, after the Eleventh Doctor showed Craig who he was, he pointed to his face and said, "Eleventh".
With regeneration also comes a change of personality. This is likely a side effect of the process of complete physical transformation, which includes an alteration of the brain chemistry andsynaptic organisation. The viewing audience sees this most often and most dramatically in the differing quirks and personality traits of the Doctor's various incarnations. However, it appears that the Doctor's core personality traits of heroism and intolerance of injustice are still retained. The Doctor also sometimes goes through a period of physical and psychological instability (which has included partial amnesia, temporary manic depression, walking into a tree and on one occasion an act of physical violence against his companion) following a regeneration, but it is not clear if this is true of all Time Lord regenerations, particularly since the Doctor's regenerations tend to happen due to stressful and violent situations. Regenerations have been known to fail, and may require assistance, technological or otherwise, or a period of recovery to successfully complete the process. The Brain of Morbius suggests that Time Lords other than the Doctor may experience difficult regenerations, since the Sisterhood of Karn had been supplying them with an "elixir of life" that could assist the process.
In some cases, future potential incarnations can achieve independent, though temporary, existence. In Planet of the Spiders, a Time Lord, K'anpo Rinpoche, creates a corporeal projection of a future incarnation which has such an existence under the name Cho Je until he regenerates into that incarnation. The Valeyard, a "distillation of the Doctor's evil side, who could potentially exist between the Doctor's twelfth and final incarnations", appears in The Trial of a Time Lord (1986) opposite the Sixth Doctor; the Valeyard is offered the Doctor's future regenerations which would "make his potential existence concrete". Another example is "The Watcher", who repeatedly appears to the Fourth Doctor in Logopolis (1981), and ultimately merges with him as part of his regeneration into his fifth incarnation.
The Time Lords' ability to change species during regeneration is referred to in the television movie by the Eighth Doctor in relation to the Master. This is supported by the implication by the Daleksthat the First Doctor's apparently human appearance was not his true form (The Daleks' Master Plan, 1965) and the Fourth Doctor's Time Lady companion Romana's regeneration scene in Destiny of the Daleks (1979). In that scene Romana demonstrates an apparent ability to "try on" different bodies from a number of different species during her regeneration, before settling on a final, humanoid form which physically resembles Princess Astra of Atrios (see discussion below).
While explaining the process of regeneration to Rose at the end of "The Parting of the Ways", the Ninth Doctor suggests that his new form could have "two heads", or even "no head", although it is unclear if he is merely joking. In the 2005 Children in Need special, which takes place immediately after, the newly regenerated Tenth Doctor, while examining his new body, makes a point of checking that he has two arms, two legs and two hands, implying that regenerations can sometimes result in physically deformed or non-humanoid forms; whether this is also a joke is not clear (or could be due to the psychological stress of regeneration). In the second part of The End of Time (2010), the Eleventh Doctor also enumerates eyes, ears, hands, fingers, and legs, and after feeling his hair, even wonders for a moment if he has changed sex. (In a later episode, "The Doctor's Wife," the Doctor refers to another Time Lord, the Corsair, having been both male and female in various incarnations.)
Whether or not Time Lords can recognise each other across regenerations is unclear from the television series. For example, in The Deadly Assassin an old classmate of the Doctor's, Runcible, is slow to recognise the Doctor in his fourth incarnation, and once he has, it then takes him a while to realise that his appearance has changed. However, in The Armageddon Factor (1978), Drax, another old classmate, recognises the Fourth Doctor immediately although they had not seen each other since the Academy (the Doctor takes a while to remember Drax, though). Shortly thereafter, in Destiny of the Daleks, the Doctor fails to recognise the yet-to-stabilize, newly-regenerated Romana. In The Twin Dilemma, the Sixth Doctor attributes old friend and fellow Time Lord Azmael's failure to recognise him to the fact that "I have regenerated twice since our last meeting." Yet in "The Sound of Drums" (2007) the Doctor states that Time Lords can "always" recognise each other, and recognises Harold Saxon as the Master on sight. However, in the mini-episode "Time Crash", the Fifth Doctor fails to recognise Tenth Doctor on first meeting, thinking he was merely "a fan" who has sneaked into the TARDIS.
It has been suggested in the series many times that regeneration is not guaranteed and can fail. After his cellular structure is decimated by the Metabelis crystals in Planet of the Spiders, the Third Doctor's regeneration requires "a little push" from fellow Time Lord K'anpo Rimpoche before it can proceed. As he succumbs to spectrox toxaemia in The Caves of Androzani, the Fifth Doctor says, "I might regenerate... I don't know... It feels different this time...". He then hallucinates, seeing his former companions encouraging him to survive, before the Master overwhelms them all, telling him he must die. The 1996 TV movie showed the Doctor's regeneration delayed for more than three hours, with the Eighth Doctor later remarking that the fact his Seventh incarnation was under anaesthesia at the time of his "death" could have "destroyed the regenerative process", and that he was "dead" prior to his regeneration. In many episodes, the Doctor doubts his own survival, though it is not always clear whether such statements refer to the death of only that particular incarnation. (The only time he makes a completely unambiguous distinction between these two scenarios – in The End of Time – he makes it clear that he regards regeneration as nearly as bad as death, because as he sees it, he dies and "a new man" walks away). In The Mind of Evilthe Master points a conventional firearm at the Doctor and threatens to "put a bullet through both [his] hearts," while in "Forest of the Dead", Professor Song warns that electrocution would stop both the Time Lord's hearts, killing him. In "Turn Left"—which presents an alternative timeline—a UNIT member speculates that the Tenth Doctor is killed "too quickly for him to regenerate." In the first part of The End of Time, the Doctor tells Wilfred Mott that if he is killed before the regeneration can start, then it will fail.
In the Series 6 premiere, "The Impossible Astronaut", a future version of the Eleventh Doctor is shot twice by the titular astronaut and begins to regenerate. However he is then shot again and dies, showing that not only can he be killed before regeneration, but also while the change is taking place. (However, the later revelation, in "The Wedding of River Song" that what was shot was in fact a mechanical avatar of the Doctor and not the real person calls this into question.) Later, in the fourth episode of the series, "The Doctor's Wife", it was stated that Time Lords can potentially change gender during a regeneration, something which had never been confirmed before (although, at certain points during the show's history, female actors have been suggested as possible candidates to play the Doctor). In "A Good Man Goes To War" Madame Vastra explained how the Time Lords became what they became through billions of years of exposure to the Time Vortex. In the series 6 episode "Let's Kill Hitler" the Doctor was revived after death by River Song who used her remaining regenerations to do this.
Subsequent regenerations retained essentially the same method, with or without additional video or make-up effects. The transition from the Fourth to the Fifth Doctor used an additional make-up effect representing a transitional form known as the Watcher, but aside from this, other regenerations in the original series run simply mixed the image of the incoming actor on top of the outgoing one. The transition from the Seventh to the Eighth Doctor in the 1996 television movie took advantage of the higher budget and modern computer animation technology to "morph" the features of Sylvester McCoy into those of Paul McGann.
With the exception of the transitions from the Second to Third and the Eighth to Ninth Doctors, each regeneration was shown on-screen, with the previous incumbent in the role symbolically "handing off" the character to the next. The Second Doctor was never seen to actually change into the Third, simply fading off into darkness at the end ofThe War Games and then stumbling out of the TARDIS, already regenerated, at the start of Spearhead from Space (1970).
The regeneration of the Sixth Doctor into the Seventh is the only time that a single actor took on the roles of two incarnations of the Doctor. Colin Baker declined the invitation to film the regeneration sequence at the start of Time and the Rani (1987) due to the circumstances in which the BBC dismissed him from the role.[5] As a result, Sylvester McCoy had to don his predecessor's costume and a blond curly wig, lying face down, with the mixing effect to the Doctor's "new" features occurring as he was turned over.
The 2005 series, which revived the programme after a 16-year hiatus, began with the Ninth Doctor already regenerated, with no explanation given as to the circumstances behind the change (although dialogue within the debut episode, Rose indicated the change had recently occurred). In the documentary series Doctor Who Confidential, producer Russell T Davies explained his reasoning that, after such a long hiatus, a regeneration in the first episode would not just be confusing for new viewers but also lack dramatic impact, as there would be no emotional investment in the character before he was replaced.[6] The regeneration of the Ninth Doctor into the Tenth at the end of "The Parting of the Ways" (2005) was seen, and also used computer effects to morphChristopher Eccleston into David Tennant. In the episode of Doctor Who Confidential accompanying the episode "Utopia", it was stated that the production team had decided that this would be a common effect for all Time Lord regenerations (the Master's in this case) rather than having a regeneration sequence chosen on a whim by the director. A similar sequence was used for David Tennant's regeneration into Matt Smith in "The End of Time", though this time the effect on the TARDIS is much more violent, possibly due to the Doctor's attempts to fight the regeneration or the severity of the radiation poisoning that brought about the process to begin with. This regeneration effect was also used in "The Impossible Astronaut" when the Eleventh Doctor is shot twice, albeit with a more wispy effect. However in this case the full transition is not seen as the Doctor is shot again fatally before completing the process. It was later revealed that this was actually a robot impersonating the Doctor. The effect was used again at the end of the following episode "Day of the Moon" when a mysterious girl also regenerated (although her new incarnation is not depicted), and in "Let's Kill Hitler" when Mels (Nina Toussaint-White) is shot and regenerates into River Song (Alex Kingston). The same effect appears when Donna Noble (Catherine Tate) and the Tenth Doctor's spare hand together spawn a Doctor-Donna hybrid in "Journey's End".
In the series The exact mechanism that makes regeneration possible is not stated in the television series, but it is generally assumed in the spin-off media that the ability to regenerate may be linked to what is known as the "Rassilon Imprimatur", the symbiotic nuclei of a Time Lord that bonds him or her to a TARDIS, and allows his or her body to withstand the molecular stresses of time travel (The Two Doctors, 1985).[7] In "The Christmas Invasion" (2005) it was stated the regenerative cycle generates a large amount of energy that suffuses the Time Lord's body. As demonstrated by theTenth Doctor, in the first fifteen hours of regeneration this energy is enough to even rapidly regrow a severed hand.[8] In the moments following his regeneration into his eighth incarnation, the Doctor possessed enough physical strength to batter a steel door completely off its hinges. In Let's Kill Hitler River Song is able to repel bullets after regenerating.
It is first stated in The Deadly Assassin (1976) that a Time Lord can regenerate twelve times before dying (thirteen incarnations in all). There are exceptions to this rule, however; when a renegade Time Lord known as the Master finds himself at the end of his regenerative cycle, he takes possession of the body of another person to continue living (The Keeper of Traken, 1981), although he was using the Source of Traken to bind his mind to the body. It may be that the Time Lords also have the ability to circumvent the limit – in The Five Doctors (1983) the Master is offered a new cycle of regenerations by the High Council in exchange for his help. The fact that the Master is inhabiting a non-Gallifreyan body at the time implies that it is possible to grant them to a non-Gallifreyan, albeit one inhabited by a Time Lord mind (although River Song is shown to have the ability to regenerate due to altered DNA that has similarities to Time Lord DNA, a side effect of being conceived in a TARDIS). In "The Sound of Drums" (2007) the Master is revealed to have been granted a new body by the Time Lords during the Time War with at least two new regenerations. Non-Gallifreyans are also seen to regenerate in Underworld (1978) and Mawdryn Undead (1983), but with adverse side effects. In Mawdryn Undead these appear to be the result of mishandling stolen technology, but in Underworld they are implied to be the inevitable result of limited technology that reinvigorates, rather than transforms, the subject's appearance (in this case, the Minyans, with whom the Time Lords shared much of their technology), thereby regenerating 'the body, not the soul'.
The BBC's Series 4 FAQ[9] suggests that now the Time Lord social order has been destroyed, the Doctor may be able to circumvent the limit on regenerations; it says: "Now that his people are gone, who knows? Time Lords used to have 13 lives." In Death of the Doctor (a 2010 The Sarah Jane Adventures serial), the Eleventh Doctor responds to a question from Clyde Langer by saying he can regenerate "507" times. Early news reports, before the episode was broadcast, suggested he would say there is no limit to the number of regenerations.[10] Writer Russell T Davies explained in an interview with SFX that the line was not intended to be taken seriously and is instead a commentary. He insisted that the "thirteen lives" rule was too deeply entrenched in the viewer consciousness for his throwaway line to affect it.[11]
In the Fourth Doctor story The Brain of Morbius, the Doctor participates in a mental ‘duel’ with another Time Lord. The machine to which their minds are connected begins to project the faces of the "losing" contestant’s regenerations in chronologically descending order. As the Doctor is overpowered by Morbius, the images change successively to those of the third, second and first Doctors, then eight further faces appear. The narrative does not definitively assert that these are past incarnations of the Doctor rather than of Morbius; other evidence from the series suggests they cannot be. The Doctor himself has numbered his regenerations on several occasions, each time indicating that the William Hartnell incarnation was the first. For example, it is explicitly stated by the Fifth Doctor in Mawdryn Undead that he has eight incarnations left, and in The Five Doctors Peter Davison's Doctor (introducing himself to the First Doctor) says that he is the fourth regeneration, meaning that there have been five of him. Again in The Five Doctors, the First Doctor refers to himself as the "original" and in Doctor Who, Paul McGann, with Sylvester McCoy on screen, states both "A Time Lord has thirteen lives and The Master'd used all of his." and "In all my travels through space and time, and nearing the end of my seventh life...". Most recently, inThe Lodger, after the Eleventh Doctor showed Craig who he was, he pointed to his face and said, "Eleventh".
With regeneration also comes a change of personality. This is likely a side effect of the process of complete physical transformation, which includes an alteration of the brain chemistry andsynaptic organisation. The viewing audience sees this most often and most dramatically in the differing quirks and personality traits of the Doctor's various incarnations. However, it appears that the Doctor's core personality traits of heroism and intolerance of injustice are still retained. The Doctor also sometimes goes through a period of physical and psychological instability (which has included partial amnesia, temporary manic depression, walking into a tree and on one occasion an act of physical violence against his companion) following a regeneration, but it is not clear if this is true of all Time Lord regenerations, particularly since the Doctor's regenerations tend to happen due to stressful and violent situations. Regenerations have been known to fail, and may require assistance, technological or otherwise, or a period of recovery to successfully complete the process. The Brain of Morbius suggests that Time Lords other than the Doctor may experience difficult regenerations, since the Sisterhood of Karn had been supplying them with an "elixir of life" that could assist the process.
In some cases, future potential incarnations can achieve independent, though temporary, existence. In Planet of the Spiders, a Time Lord, K'anpo Rinpoche, creates a corporeal projection of a future incarnation which has such an existence under the name Cho Je until he regenerates into that incarnation. The Valeyard, a "distillation of the Doctor's evil side, who could potentially exist between the Doctor's twelfth and final incarnations", appears in The Trial of a Time Lord (1986) opposite the Sixth Doctor; the Valeyard is offered the Doctor's future regenerations which would "make his potential existence concrete". Another example is "The Watcher", who repeatedly appears to the Fourth Doctor in Logopolis (1981), and ultimately merges with him as part of his regeneration into his fifth incarnation.
The Time Lords' ability to change species during regeneration is referred to in the television movie by the Eighth Doctor in relation to the Master. This is supported by the implication by the Daleksthat the First Doctor's apparently human appearance was not his true form (The Daleks' Master Plan, 1965) and the Fourth Doctor's Time Lady companion Romana's regeneration scene in Destiny of the Daleks (1979). In that scene Romana demonstrates an apparent ability to "try on" different bodies from a number of different species during her regeneration, before settling on a final, humanoid form which physically resembles Princess Astra of Atrios (see discussion below).
While explaining the process of regeneration to Rose at the end of "The Parting of the Ways", the Ninth Doctor suggests that his new form could have "two heads", or even "no head", although it is unclear if he is merely joking. In the 2005 Children in Need special, which takes place immediately after, the newly regenerated Tenth Doctor, while examining his new body, makes a point of checking that he has two arms, two legs and two hands, implying that regenerations can sometimes result in physically deformed or non-humanoid forms; whether this is also a joke is not clear (or could be due to the psychological stress of regeneration). In the second part of The End of Time (2010), the Eleventh Doctor also enumerates eyes, ears, hands, fingers, and legs, and after feeling his hair, even wonders for a moment if he has changed sex. (In a later episode, "The Doctor's Wife," the Doctor refers to another Time Lord, the Corsair, having been both male and female in various incarnations.)
Whether or not Time Lords can recognise each other across regenerations is unclear from the television series. For example, in The Deadly Assassin an old classmate of the Doctor's, Runcible, is slow to recognise the Doctor in his fourth incarnation, and once he has, it then takes him a while to realise that his appearance has changed. However, in The Armageddon Factor (1978), Drax, another old classmate, recognises the Fourth Doctor immediately although they had not seen each other since the Academy (the Doctor takes a while to remember Drax, though). Shortly thereafter, in Destiny of the Daleks, the Doctor fails to recognise the yet-to-stabilize, newly-regenerated Romana. In The Twin Dilemma, the Sixth Doctor attributes old friend and fellow Time Lord Azmael's failure to recognise him to the fact that "I have regenerated twice since our last meeting." Yet in "The Sound of Drums" (2007) the Doctor states that Time Lords can "always" recognise each other, and recognises Harold Saxon as the Master on sight. However, in the mini-episode "Time Crash", the Fifth Doctor fails to recognise Tenth Doctor on first meeting, thinking he was merely "a fan" who has sneaked into the TARDIS.
It has been suggested in the series many times that regeneration is not guaranteed and can fail. After his cellular structure is decimated by the Metabelis crystals in Planet of the Spiders, the Third Doctor's regeneration requires "a little push" from fellow Time Lord K'anpo Rimpoche before it can proceed. As he succumbs to spectrox toxaemia in The Caves of Androzani, the Fifth Doctor says, "I might regenerate... I don't know... It feels different this time...". He then hallucinates, seeing his former companions encouraging him to survive, before the Master overwhelms them all, telling him he must die. The 1996 TV movie showed the Doctor's regeneration delayed for more than three hours, with the Eighth Doctor later remarking that the fact his Seventh incarnation was under anaesthesia at the time of his "death" could have "destroyed the regenerative process", and that he was "dead" prior to his regeneration. In many episodes, the Doctor doubts his own survival, though it is not always clear whether such statements refer to the death of only that particular incarnation. (The only time he makes a completely unambiguous distinction between these two scenarios – in The End of Time – he makes it clear that he regards regeneration as nearly as bad as death, because as he sees it, he dies and "a new man" walks away). In The Mind of Evilthe Master points a conventional firearm at the Doctor and threatens to "put a bullet through both [his] hearts," while in "Forest of the Dead", Professor Song warns that electrocution would stop both the Time Lord's hearts, killing him. In "Turn Left"—which presents an alternative timeline—a UNIT member speculates that the Tenth Doctor is killed "too quickly for him to regenerate." In the first part of The End of Time, the Doctor tells Wilfred Mott that if he is killed before the regeneration can start, then it will fail.
In the Series 6 premiere, "The Impossible Astronaut", a future version of the Eleventh Doctor is shot twice by the titular astronaut and begins to regenerate. However he is then shot again and dies, showing that not only can he be killed before regeneration, but also while the change is taking place. (However, the later revelation, in "The Wedding of River Song" that what was shot was in fact a mechanical avatar of the Doctor and not the real person calls this into question.) Later, in the fourth episode of the series, "The Doctor's Wife", it was stated that Time Lords can potentially change gender during a regeneration, something which had never been confirmed before (although, at certain points during the show's history, female actors have been suggested as possible candidates to play the Doctor). In "A Good Man Goes To War" Madame Vastra explained how the Time Lords became what they became through billions of years of exposure to the Time Vortex. In the series 6 episode "Let's Kill Hitler" the Doctor was revived after death by River Song who used her remaining regenerations to do this.