Regeneration

Regeneration was the process by which Time Lords renewed themselves, causing a complete physical and often psychological change. It could happen because of severe illness (TV: The Tenth Planet, Planet of the Spiders, The Caves of Androzani, The End of Time, Day of the Moon) or injury. (TV: Logopolis, Time and the Rani, Doctor Who, Utopia, Let's Kill Hitler) It could also be invoked by choice, whether voluntary or involuntary. (TV: The War Games, Destiny of the Daleks) Conversely, regeneration could be stopped by choice. (TV: Last of the Time Lords)

How regeneration worked
Different explanations were given for the process of regeneration.

One theory held that Cardinal Rassilon had been investigating a method of regenerating decayed and diseased tissue via a series of self-replicating, biogenic molecules. The cells of a Gallifreyan body would be repaired, restored and re-organised, resulting in a wholly new physical form. The brain cells would also be rearranged, though to a lesser extent; the new incarnation would retain the memories and personality of the former incarnation. Rassilon intended this mechanism only for the Gallifreyan elite. He also inputted a parameter of twelve regenerative cycles to avoid decaying biogenic molecules. (AUDIO: Zagreus)

Another explanation stated that Time Lords had triple-helix DNA: the third strand was added by Rassilon to enable regeneration. (PROSE: The Crystal Bucephalus)

Exposure over billions of years to the Untempered Schism was implied to have contributed to the Time Lords' ability to regenerate. (TV: A Good Man Goes to War)

Details
Time Lords released massive amounts of a hormone called lindos in moments of extreme trauma, and it was this hormone which triggered regeneration. Newly regenerated Time Lords could be identified by elevated levels of lindos in their system. (PROSE: The Twin Dilemma, AUDIO: Unregenerate!)

Time Lords were also said to have "packets" of regeneration energy in their bodies, one for each life. These packets could be physically removed from a Time Lord's body, essentially robbing them of their regenerations. (TV: Mawdryn Undead)

Appearance
During a regeneration, a Time Lord's body shone with milky white light, a swirl of rainbow colours, crackled with electricity (TV: The Tenth Planet, TV: Logopolis, Time and the Rani, Doctor Who) or with a violent-seeming discharge of bio-energy that damaged anything close to it. (TV: The Parting of the Ways, Utopia, The Stolen Earth, The End of Time, Day of the Moon, Let's Kill Hitler). In other cases, there was no visible energy discharge, just a fade away to the next incarnation. (TV: Planet of the Spiders, Destiny of the Daleks)

Just before his regeneration, the Ninth Doctor warned Rose Tyler to keep away from him. (TV: The Parting of the Ways) The Tenth Doctor did likewise shortly before his aborted regeneration during the 2009 Dalek invasion of Earth. (TV: The Stolen Earth) The damage to the TARDIS from the Tenth Doctor's regeneration into the Eleventh, enough to force a complete reconstruction into a different design, may have been an example of the dangers the regeneration energy could pose. (TV: The End of Time) However, other regenerations occurred with other individuals in close proximity. These are times when the energy from the Doctor's body wasn't as violent. (TV: The Tenth Planet, Planet of the Spiders, Logopolis, The Caves of Androzani, Time and the Rani)

Physical and mental change
During regeneration, there were the genetic equivalent of 'bit errors' in the DNA of the regenerating cells. The Time Lord would change in appearance, height, mass or apparent age. The personality would also change; even the cells and chemistry of the brain regenerated. Although the aspects of their personality caused by "nurture," their basic personality, would not change, the "nature," their aspects, quirks, and habits, contribution to their personality would. (AUDIO: The Sirens of Time) One source stated that Time Lords of the Oldblood Houses, born with just one heart, grew a second heart on regeneration. This included the Doctor, who in his first incarnation had only one heart. (PROSE: The Man in the Velvet Mask) According to the Eleventh Doctor, every regeneration was painful. (TV: Death of the Doctor)

More extreme changes were possible. Cavisadoratrelundar regenerated a complete body after being decapitated; the process was cut short when he was stabbed through both hearts. (PROSE: The Shadows of Avalon) The Eleventh Doctor feared he had become a woman, (TV: The End of Time) while the Ninth Doctor suggested two heads or none were possible (TV: The Parting of the Ways). His eleventh incarnation stated clearly that he could become "anything". (TV: Death of the Doctor) The Corsair was a Time Lord/Time Lady whom the Doctor had known in both sexes. (TV: The Doctor's Wife) Time Lords could also change their skin colour as seen in Melody Pond's case when she regenerated into her third incarantion. (TV: Let's Kill Hitler)

In nearly all cases, Time Lord regenerations remained humanoid; one Time Lord, Lord Cardinal Zero, regenerated into an avian lifeform. (AUDIO: Spring) Though a healthy body seems to be the default, the Doctor's eleventh incarnation made it a priority - even amidst serious damage to his TARDIS - to immediately conduct a physical inventory to make sure he still had two legs and sufficient fingers, eyes, ears, a nose, chin and hair. (TV: The End of Time)


 * The Doctor's ability to regrow a severed limb soon after regeneration (TV: The Christmas Invasion) suggests the possibility that had the eleventh incarnation, for example, discovered a deficiency - a missing leg, for example - he might have been able to rectify the situation.

After-effects
During the first few hours of the regeneration, the Time Lord might suffer from confusion, erratic behaviour, extended periods of unconsciousness or memory loss. Motor control could be impaired, (the Eleventh Doctor expressed difficulty "steering" his new body), and a Time Lord could suffer random spasms as the regeneration settled. (TV: The Eleventh Hour) The Doctor, in particular, seemed extremely susceptible to side-effects from the regenerative process. His third incarnation was incapacitated for some time at Ashbridge Cottage Hospital (TV: Spearhead from Space). Likewise, the fourth incarnation of the Doctor suffered acute delirium and memory loss and was placed under bed rest at UNIT Headquarters. (TV: Robot) It took some time for his newly-regenerated fifth incarnation to remember his own identity. (TV: Castrovalva) Following another regeneration, his sixth incarnation tried to strangle Peri to death before re-asserting control of himself (TV: The Twin Dilemma), and in his following one, the Seventh Doctor was sufficiently weakened that the the Rani was able to effectively brainwash him. (TV: Time and the Rani) His seventh regeneration was particularly traumatic for the Eighth Doctor, combining amnesia and emotional instability. (TV: Doctor Who) On two occasions the Doctor crashed the TARDIS following regeneration, first when the Tenth Doctor compulsively sped up the TARDIS and hit it against the wall of the Powell Estate, then when the Tenth Doctor's regeneration into the Eleventh damaged the TARDIS, causing it to crashland into a storage shed in Leadworth. (TV: Children in Need Special, The Christmas Invasion, The End of Time, The Eleventh Hour) After his tenth regeneration, the Eleventh Doctor experienced powerful, rapidly alternating food cravings, declaring a certain type of food his favourite one minute, and saying he hated it the next. (TV: The Eleventh Hour)

A Zero Room could help with the process, as it removed all outside distractions. (TV: Castrovalva) After his first regeneration, the Second Doctor implied that the TARDIS itself helped the process along. (TV: The Power of the Daleks) For other species, a metamorphic symbiosis regenerator might offer help when regenerations failed, though whether this true of Time Lords is unknown. (TV: Mawdryn Undead)

Were a Time Lord knocked unconscious, the whole process might start all over again (PROSE: The Power of the Daleks), but this was not a certainty. (TV: The Eleventh Hour)

After a while, the Time Lord's body would settle down, though they could regrow limbs within fifteen hours of the regeneration due to residual energy. (TV: The Christmas Invasion) Even after the physical transformation, changes might occur. The Doctor was excessively tired after his third regeneration, falling asleep in many odd locations. (TV: Robot) The Fifth Doctor's hair went from longer to shorter to longer in the space of a few days. (PROSE: Cold Fusion) For a short time after regenerating, a Time Lord displayed greater strength than usual; The Doctor's fourth incarnation was able to karate-chop a brick in half while recovering from his regeneration (TV: Robot), his eighth incarnation broke down a steel door bare-handed immediately following his regeneration. (TV: Doctor Who) Melody Pond, following her final regeneration, not only used her regenerative energy to survive a hail of gunfire by Nazi soldiers, but channelled it into a focused blast of energy that knocked out the entire squad. (TV: Let's Kill Hitler)

However, some Time Lords were able to regenerate with little or no overt complications, as in the case of Romana. (TV: Destiny of the Daleks)

Limitations
Though Time Lords could regenerate after severe injuries, regeneration was by no means guaranteed. Maxil implied that a fatal blast from a staser (an energy weapon used by the Chancellory Guard on Gallifrey) could prevent regeneration. (TV: Arc of Infinity) Stabbing or shooting a Time Lord through both hearts at the same time (PROSE: The Shadows of Avalon, PROSE: World Game), burning out both hearts (TV: Forest of the Dead) or drowning, if it happened quickly enough (TV: Turn Left) could also end a Time Lord's life regardless of how many regenerations they had left. Acid could also destroy the ability to regenerate. (PROSE: Night of the Humans) Some illnesses, such as Chen-7, could preclude regeneration. (TV:The Girl Who Waited) Certain poisons could prevent the regeneration process, such as the poisoned lipstick River Song possessed. (TV: Let's Kill Hitler) As well, the application of various medications, such as general anaesthetics from Earth, were known to disrupt or destroy the regenerative process. (TV: Doctor Who)

Ideally, regeneration would be undergone within a low-grade telepathic field. The presence of another Time Lord was recommended to assist with any difficulties, and the newly-regenerated Time Lord best remained in a state of total tranquility for a time afterward to allow the mind and body to properly readjust. (PROSE: Cold Fusion)

Regenerative cycle
Time Lords had a limited regenerative cycle of twelve regenerations, consisting of thirteen incarnations, after which they would suffer permanent death. (TV: The Deadly Assassin, Doctor Who) Time Lords could will themselves to die by regenerating when they had no more regenerations left to use, as Azmael chose to do. (TV: The Twin Dilemma) Rassilon apparently had physical reasons to impose this restriction. (AUDIO: Zagreus)

As with most such "rules", there were occasionally exceptions to the twelve regeneration limit. The High Council offered the Master a new regenerative cycle if he rescued the various incarnations of the Doctor from the Death Zone. (TV: The Five Doctors) Although he did not receive this award on that occasion, during the Last Great Time War the Master was given the ability to regenerate at least once more. (TV: Utopia)

In his Eleventh incarnation, the Doctor claimed he could regenerate five hundred and seven times. (TV: Death of the Doctor) Whether the Doctor was speaking the truth is unknown. Though since the Time Lords are gone now, it is possible the limitations are moot.

Time Lords could be revived with regeneration energy. Doing this, Melody Pond (in her third incarnation) used up her remaining regenerations to save the Doctor's life and was hospitalised as a result. It is unknown if this act gave the Doctor all her remaining regenerations. (TV: Let's Kill Hitler) The Doctor was later able to use this to heal River Song's broken wrist which angered her. (TV: ''The Angels Take Manhattan)

Control over regeneration
Generally, the regeneration process triggered itself when a Time Lord was too badly injured to survive; however, in some cases, Time Lords exercised control over the process. Romana seemed to regenerate on a whim (TV: Destiny of the Daleks), while Azmael began a thirteenth regeneration to end his life. (TV: The Twin Dilemma)

The degree of control that Time Lords had over their end appearance was unclear. The Master made his next regeneration as young as the Tenth Doctor appeared at the time, though, since most regenerations led to younger bodies, this may have been a coincidence (TV: Utopia). Melody Pond announced she was "focusing on a dress size" in her final regeneration. (TV: Let's Kill Hitler) Romana seemed adept enough at the process to custom design her new form, trying several bodies before finally deciding on a copy of Princess Astra. The Doctor criticised Romana for taking on the form of another person, suggesting such things were not unheard of. (TV: Destiny of the Daleks) In contrast to Romana, the Doctor did not seem to have much control over his post-regeneration appearance; after his fourth regeneration, he commented "That's the problem with regeneration, you never quite know what you're going to get." (TV: Castrovalva) He would restate this sentiment immediately prior to his ninth regeneration. (TV: The Parting of the Ways)
 * However as the Doctor's regenerations have thus far either been forced on him or been triggered when an incarnation has been mortally wounded, it may be that the appearance of the next incarnation can be controlled if the regeneration is voluntarily triggered (as was Romana's).


 * Melody Pond commented, after her regeneration, that she could make herself gradually younger over her life. Whether this was part of the regenerative process or not is unknown.

While skilled Time Lords could choose their new form with a voluntarily induced regeneration, the process could go horribly wrong and leave the Time Lord in a severely misshapen body. This problem might be exacerbated by the Time Lord immediately starting another regeneration instead of obtaining medical assistance, amplifying the defects in the regeneration. The end result of these abortive regenerations was inevitably a mutated monstrosity that could only be put out of its misery by complete disintegration. (PROSE: The Twin Dilemma)

Some Time Lords were capable of momentarily regenerating, or partially regenerating. Though this could use up a lot of regenerative energy, it would give the Time Lord a new set of genes, allowing them to fool genetic sensors. The Seventh Doctor used this method on the planet Purgatory to fool the genetic scanner used by the Landsknechte. (PROSE: Original Sin)

The Time Lords were apparently capable of controlling the regeneration of individual Time Lords, either forcing a regeneration, influencing the new appearance (TV: The War Games), or removing later regenerations. (TV: The Ultimate Foe)

With effort, Time Lords could resist regeneration, effectively committing suicide. (PROSE: The Power of the Daleks) The Master did so after being shot, ostensibly to avoid becoming the Tenth Doctor's eternal prisoner. (TV: Last of the Time Lords) Similarly, the Fifth Doctor once threatened System with resisting regeneration to stop the device from learning the biological details of the act. (AUDIO: The Gathering) This was not always an option, however, as the Doctor noted fearfully that while his companion could die only once, he might repeatedly regenerate and live out all of his lives when the TARDIS stalled in space. (TV: Vengeance on Varos) The Tenth Doctor was able to delay his regenerative process long enough to revisit each of his former companions of his own incarnation and his past selves as well. (TV: The End of Time, TV: Death of the Doctor)

Regenerative difficulty
While most regenerations seemed to cause moments of mental instability, with temporary amnesia often noted, some offered particularly profound instances of physical peril. The Fifth Doctor feared that his regeneration "was failing" when he found himself reverting to previous personas, and could only be righted with the use of the TARDIS Zero Room. Ambient complexity may also contribute to the failure of a regeneration. (TV: Castrovalva) The Eighth Doctor claimed that anaesthesia had "nearly destroyed the regenerative process" during his seventh regeneration as an explanation for the particularly severe amnesia he suffered afterwards. (TV: Doctor Who) During the Tenth Doctor's post regenerative state, he suffered an arrest in one of his hearts and began to exhale regenerative energy when Rose Tyler revived him too early. After this he slipped into a coma-like state for most of a day (TV: The Christmas Invasion). When the Tenth Doctor underwent his own regeneration, the process was exceptionally violent and destructive to the TARDIS, likely a consequence of his delay of the procedure and/or the massive amounts of radiation he absorbed which caused the regeneration to start in the first place, though it hasn't been clear whether the severity of a regeneration-causing injury impacts the violence of the regeneration or not (TV: ''The End of Time). ''

Regeneration, especially later ones, could be painful. Melody Pond screamed during one regeneration, (TV: Let's Kill Hitler), as did the Master during his transition into his "Harold Saxon" incarnation. (TV: Utopia). Both Ninth and Tenth Doctors also appeared to grimace in pain during the process (TV: The Parting of the Ways, The End of Time).

Reversal


There were many ways to reverse a regeneration. One way involved the sacrifice of another, causing the regeneration to reverse. One example of this was when the Third Doctor had an encounter with The Nurazh. As the Doctor battled the Nurazh's main host, the two fell off a building, killing the Doctor. As the Third Doctor regenerated into the Fourth, the Nurazh possessed the Time Lord's body, however, it found itself unable to cope with the two Time Lord minds within the body and it soon perished, restoring the Doctor to his previous incarnation in the process. (PROSE: The Touch of the Nurazh)

The regeneration process could also be delayed to allow healing. The Second Doctor was shot in the head when confronted by guards on Skybase, causing damage to his skull and frontal lobe; and the subsequent fall broke his nose, jaw, right femur, and collarbone, along with some spine damage. He began to regenerate, however, an injection of Shiner DNA delayed the regeneration and kept him alive long enough for his body to go into a six-month healing coma to recover on its own. (PROSE: The Indestructible Man)

A Time Lord could prevent death and regeneration by focusing the regenerative energies into a severed appendage, like the Doctor's hand. His hand siphoned off the excess energy that would have changed his appearance while the Tenth Doctor used just enough to heal himself from the injury sustained from a Dalek Gunstick. This resulted in the appendage storing enough energy to actually grow an identical Time Lord when it came in contact with Donna Noble. As a consequence, the Doctor appeared to regenerate and heal but did not change. (TV: Journey's End)

Cross-species transformations
I.M. Foreman, a Gallifreyan (but not a Time Lord), absorbed the DNA around him and underwent indescribable changes as a result of mutations, transcending sex, species and even physical existence itself. (PROSE: Interference - Book One, Interference - Book Two) Romana, prior to her regeneration into her second incarnation, appeared to have taken on a near-human blue-skinned form. (TV: Destiny of the Daleks) However, one account held that the TARDIS itself, rather than Romana, adopted this shape. (PROSE: The Lying Old Witch in the Wardrobe)

Aborted regeneration
Occasionally, a regeneration would fail and the regeneration would abort. Though the Time Lord would have regenerated, they would be severely deformed. Though Time Lord technology could treat this, on some occasions the damage would be too severe to fix.

After being shot by the War Lords, the War Chief was barely able to survive. While being returned to the War Lords' planet, his body attempted to regenerate. Due to the massive injuries and the lack of medical care, this regeneration aborted. This resulted in two conjoined individual bodies, poorly fused together (PROSE: Timewyrm: Exodus)

Attitude toward regeneration
As noted above, regeneration was not guaranteed. The Doctor on numerous occasions believed he was at risk of actually dying. Even with regeneration a possibility, the Doctor came to feel such a change as being a "death". In recollecting the events surrounding the Master's attempt to steal the Eye of Harmony, the Eighth Doctor referred to his incarnations as "lives". (TV: Doctor Who) In fact, the Doctor seemed to regard his previous incarnations as different individuals, capable of interacting and working with each other. (TV: The Three Doctors, The Five Doctors, The Two Doctors, Time Crash) Time Lords such as the War Chief were unconcerned about wasting regenerations, while others such as the Doctor warned not to waste them. (PROSE: Invasion of the Cat-People) Iris Wildthyme once confided in Samantha Jones that regeneration was treated on Gallifrey the same way sex was on Earth. (PROSE: The Scarlet Empress)

The Doctor's attitude towards regeneration seemed to change during his later incarnations, considering it more like true death. In his ninth incarnation, the Doctor bade farewell to his companion, ("I'm not gonna see you again. Not like this.") even though he was not actually dying. (TV: The Parting of the Ways) The Doctor's tenth incarnation was concerned about a prediction made regarding his own impending regeneration, saying "Even if I change, it feels like dying. Everything I am dies. Some new man goes sauntering away...and I'm dead." (TV: The Waters of Mars, The End of Time)

Despite The Doctor's attitude toward regeneration, both Harriet Jones and Sarah Jane Smith felt the same way about The Doctor throughout his incarnations (TV:The Christmas Invasion, TV:Death of the Doctor), with Harriet Jones stating that he was "absolutely the same man, still believing in this despite the Doctor threatening to destroy her government, after she ordered Torchwood to blow up the Sycorax spaceship. (TV:The Christmas Invasion)

Regeneration in other species
On several occasions, individuals of other species (or even artificial life) were capable of regeneration. In most cases, this ability was copied from the Time Lords.


 * The crew of the Minyans' ship, the P7E, could regenerate indefinitely, likely as a result of the interference by the Time Lords in their early history. Over time, they wearied of life. (TV: Underworld)
 * Mawdryn and his followers, who had stolen the Time Lords' regeneration technology, also had an apparently limitless number of incarnations, though they had no control over when it would happen and what form, often grotesque, they would change into. Consequently, they also longed for death, making their mutations a kind of de facto punishment by the Time Lords for stealing their technology. (TV: Mawdryn Undead)
 * Chris Cwej was regenerated by force to survive radiation poisoning. (PROSE: Tears of the Oracle)
 * K9 Mark I regenerated by use of a Regeneration unit after self-destructing to defeat a group of Jixen Warriors. (TV: Regeneration) He later regenerated again after exhausting his power core to defeat the Trojan. (TV: The Eclipse of the Korven)
 * The Cinder was a sentient spark which grew into a Phoenix made of living flame. When extinguished, it slowly regenerated from a single surviving spark back into its Phoenix form. (AUDIO: Frostfire)

Why regeneration?
Regeneration was introduced to the mythos of Doctor Who to solve a practical staffing problem: the production team needed to find a way to exit William Hartnell but still keep the show running.

The original idea for this replacement came from producer John Wiles and script editor Donald Tosh. They proposed to write out Hartnell during The Celestial Toymaker, a serial they commissioned and prepped, but ultimately didn't produce. Their notion was that the Celestial Toymaker would make the Hartnell Doctor disappear, but when the Doctor re-appeared he would magically be another actor entirely. (REF: The First Doctor Handbook, The Second Doctor Handbook) Though not at all a regenerative process as the term has since come to be understood, Wiles and Tosh do at least get some credit for being the first people to moot the possibility of carrying on the show with a new lead —and for the necessity of finding a narrative explanation for this switch. Though this is taken for granted today, this was an important milestone on the way to regeneration. Doctor Who could just as easily gone down the route of another 1960s show, Bewitched, where a main character was simply recast without narrative explanation.

However, Wiles and Tosh were ultimately unsuccessful in their bid to replace Hartnell, due to resistance from BBC Head of Serials, Gerald Savory. (REF: The Second Doctor Handbook) This failure was a part of the reason Wiles resigned relatively soon after taking over the show from Verity Lambert. (REF: The First Doctor Handbook)

His successor, Innes Lloyd, was better able to negotiate Hartnell's departure, in part because the climate within the BBC hierarchy had changed with Shaun Sutton's management elevation. (REF: The First Doctor Handbook) Still, it is uncertain who, precisely, came up with the idea of regeneration-as-biologic-process, rather than the mystical solution Wiles had earlier mooted. Howe, Stammers and Walker believe "the likelihood is that it emerged in discussion between Lloyd and his story editor Gerry Davis" — along with additional input from Shaun Sutton, and Kit Pedler. (REF: The Second Doctor Handbook)

Narrative origins
The earliest-known production office-generated document on the subject describes it thus:"The metaphysical change which takes place every 500 or so years is a horrifying experience — an experience in which he re-lives some of the most unendurable moments of his long life, including the galactic war [which was believed, at this time, to have been the cause of the Doctor and Susan's departure from their home planet]. It is as if he has had the LSD drug and instead of experiencing the kicks, he has the hell and dank horror which can be its effect."

- 1966 production note, entitled "The New Doctor Who", The Second Doctor Handbook p.24

Initially, the concept wasn't called "regeneration" at all, but rather "renewal". In fact, the term, so familiar to Doctor Who fans today, didn't appear until the Doctor's third regeneration, first seen by fans in 1974's Planet of the Spiders. Since The War Games, Troughton's final story, merely had the Time Lords suggesting that they would "change [his] appearance", the only explanation of regeneration — for the show's first twelve years — was found in a cryptic exchange in part one of The Power of the Daleks:


 * Ben: Now look, the Doctor always wore this. So if you're him, it should fit now, shouldn't it?
 * Ben grabs the Doctor's hand, and slips the signet ring on. But the ring, too big for the new Doctor's finger, falls to the TARDIS floor.
 * Ben: There - that settles it.
 * Doctor: I'd like to see a butterfly fit into a chrysalis case after it's spread its wings.
 * Polly: Then you did change!
 * Doctor: Life depends on change -- and renewal.
 * Ben: (sarcastically) Oh so that's it. You've been renewed have ya?
 * Doctor: (considering the notion seriously) I've been renewed, have I? That's it, I've been renewed! It's part of the TARDIS. Without it, I couldn't survive.

Although the Second Doctor's last claim of a connection between the TARDIS and regeneration has never been explored in detail, it is heavily suggested by later regeneration stories. Notably, the Third and Fourth Doctor's highest initial priority is returning to the TARDIS; (TV: Spearhead from Space, Robot) the Fifth Doctor desperately needs the TARDIS' Zero Room to assist with his regeneration; (TV: Castrovalva) and the Eighth Doctor's post-regeneration amnesia is instantly resolved when Chang Lee opens the TARDIS' Eye of Harmony. (TV: Doctor Who) The connection between man and vehicle was made explicitly clear by the visual effects in The Parting of the Ways. There, the effect used for regenerative energy was the same as the energy that emanated from, and was returned to, the heart of the TARDIS. In other words, the TARDIS-generated energy that killed the Ninth Doctor was also apparently that which revived him as the Tenth.

Tweaking regeneration
Beginning with the regeneration that resulted in the Fourth Doctor, each successive regeneration reveals a bit more about the mystery of the act.

Planet of the Spiders shows viewers that one Time Lord can help another by giving the process "a little push". This act of "gifting" regenerative energy is later expanded upon in Mawdryn Undead and Let's Kill Hitler. Both these stories take Cho Je's "push" one step further by suggesting that regenerations can be outright gifted from one being to another.

The "Cho Je push" is also tweaked a bit for the Doctor's fourth regeneration. In Logopolis the audience is introduced to a kind of "mid-regeneration Doctor", a being called "the Watcher" who exists between the Fourth and Fifth Doctors. Though his presence is never adequately explained, he is apparently corporeal enough to pilot the TARDIS and appear to characters other than the Doctor. He then combines with the dying Fourth Doctor to become the Fifth.

The notion that there is an existence for the Doctor within the act of regeneration is again mooted by AUDIO: Winter, which takes place almost entirely in that interim between incarnations. Moreover, another "intra-regenerational" version of the Doctor is seen in The Trial of a Time Lord. The story's chief antagonist is said to be the Doctor between his twelfth and thirteenth lives.

Another novelty of the fourth regeneration is the introduction of the idea that a regeneration can "fail", resulting in the Doctor's death. (TV: Castrovalva) But if the fourth regeneration focuses on a physical crisis, the next three surely stress the mental hardships of the act. The fifth regeneration leads to a kind of mania never before experienced by the Doctor. It even shakes loose some criminal tendencies. (TV: The Twin Dilemma) The next two regenerations cause temporary amnesia. (TV: Time and the Rani, Doctor Who) (Actually, the Seventh Doctor initially appears to suffer very little from post-regenerative effects in Time and the Rani; it's only after the Rani drugs him that he suffers from amnesia.) This condition is particularly profound in the newly-arrived Eighth Doctor, who completely forgets all of his past history for a number of hours. This regeneration also brings forth the notion that the Doctor actually dies prior to the metamorphosis of regeneration. The idea that the Doctor dies, even if briefly, is something that the Tenth Doctor later explains to Wilfred Mott in The End of Time part one.

The ninth regeneration, whose after-effects are documented in The Christmas Invasion, introduces the notion that the regenerative cycle lasts for fifteen hours. Within that window, the Doctor can lose body parts and yet re-grow them as he does with a hand he loses in battle with a Sycorax. Both Invasion and the preceding mini-episode also add another wrinkle to the mythos of regeneration. They show that the Doctor needs to expel regenerative energy in the aftermath of a change — something seen again in The Eleventh Hour.

The Doctor's tenth regeneration is shown to be tangibly explosive, something that hadn't been explored by any previous BBC Wales regeneration. That is, regenerative energy is depicted as being able to physically damage things. By the end of the cycle, the Doctor's TARDIS is itself in need of a "regeneration." It is implied that this explosive regeneration is due to the Doctor delaying it while he travelled to see all his former companions for an unknown period of time, thus allowing that regenerative energy to build up. (TV: The End of Time, The Eleventh Hour, TV: Death of the Doctor)

Aspects of both the ninth and tenth regenerations are invested in River Song's second regeneration, seen in Let's Kill Hitler. River Song practically begs to be shot by Nazi soldiers immediately after regeneration so that she can re-trigger her explosive regenerative energy and hurt them. The Hitler regeneration also definitively proves that skin colour can change through regeneration — though this had actually been practically settled long before by the "blue option" seen in Romana's Destiny of the Daleks regeneration.

Some details about regeneration are given by stories that don't technically feature a regeneration. For instance, The Doctor's Wife establishes that Time Lords, or at the very least the Corsair, can change gender through regeneration.

It's also shown that The Doctor's companions can influence his next incarnation's personality. Hinted at, when the Tenth Doctor told Rose to stay with the Meta-Crisis Tenth Doctor, stating he was as angry and vengeful as he was, when he first met Rose. (TV:Journey's End)

Non-narrative explanations
Because of a relative lack of narrative explanation about regeneration, some writers of non-fictional or reference books about Doctor Who have tied to fill in the gap. One theory from such a source is that regeneration is caused by a "nanomolecular virus" that rebuilt the body much like the "self-replicating biogenic molecules". (REF: The Gallifrey Chronicles) This theory has not been repeated elsewhere, however.

That regenerative look
Each new regeneration was also radically different from the previous one, even in terms of the visual effects used to represent the moment of regeneration.

Do you remember the first time?
The very first regeneration was devised and executed by vision mixer Shirley Coward, who had rather unexpectedly come up with a method of achieving the effect electronically.

The original plan of the production team was simply that William Hartnell would fall to the floor at the end of The Tenth Planet and pull his cape over his face. Troughton would then appear at the top of The Power of the Daleks, retracting the cloak. Coward's then-innovative vision mix necessitated that Troughton be hastily contracted for The Tenth Planet, part four. The series' first regeneration sequence was then duly recorded on 8 October 1966, with the cliffhanger resolution filmed two weeks later on 22 October. (REF: The Second Doctor Handbook)

Later regenerations
Each subsequent regeneration was then filmed in a variety of different ways, as dictated by the director on that particular episode. Indeed, no two regenerations were particularly similar until the Russell T Davies era.

Only BBC Wales Doctor Who attempted to standardise the way regeneration looked. With The Parting of the Ways came what is now the standard "golden glow explosion". The subsequent Children in Need Special established that there was residual "regeneration energy" after a transformation which had to be expelled through the mouth. This was seen again in The Christmas Invasion, The Eleventh Hour and The Doctor's Daughter — though the latter narrative never made quite clear that Jenny actually regenerated.

This visual standardisation has allowed narratives to play around with regeneration. The mere presence of "regeneration energy" can now be used to heighten dramatic tension. This visual short cut, unavailable to production teams in the classic era, has been a particular favourite of Steven Moffat, who used the "golden glow" liberally throughout the 2011 series. About a quarter of the 2011 episodes used that VFX in a way that wordlessly suggested regeneration.

Fan fare
Fans have long speculated as to whether the Doctor could change sex or skin colour as a result of a regeneration. They've also long speculated on the number of times that a Time Lord can regenerate, since both Doctor Who and The Sarah Jane Adventures have given different impressions on separate occasions.

Gender
In Interference - Book One and Book Two, the Time Lord I.M. Foreman was portrayed as having changed sex as a result of regeneration, though the character is noted as having received the gift of regeneration when the process was still experimental and unstable. Female versions of the Doctor appeared in the non-canonical The Curse of Fatal Death and in the Doctor Who Unbound story Exile. In the audio drama The Two Irises, Iris Wildthyme encounters a future, male incarnation of herself. In The End of Time the newly-regenerated Doctor shouts out "I'm a girl!", however this was just seconds after regeneration and it is possible this was post-regenerative trauma. In The Doctor's Wife the Doctor mentioned his friend, the Corsair, who had regenerated as both male and female.

Skin colour
In TV: Death of the Doctor the Eleventh Doctor noted that his racial characteristics were not limited to white; he "can be anything." This was the first reference to such a possibility on-screen, but not in licensed fiction. Rassilon has been portrayed by white actors Richard Mathews and Timothy Dalton on-screen while black actor Don Warrington was Rassilon's voice actor and cover-art model in Big Finish Doctor Who audio stories. In TV: Let's Kill Hitler, Melody Pond's second incarnation is black, while her first in TV: The Impossible Astronaut / Day of the Moon and A Good Man Goes to War and third, appearing throughout series 4, 5 and 6, are white.

Number
Russell T Davies noted how firmly the concept of limiting Time Lords to thirteen lives, introduced in The Deadly Assassin, was lodged in fans' minds. Davies deliberately broke the limit in Death of the Doctor, though he admits that fandom may resist his attempt to alter the programme's mythos. "When they came [to America] to launch The Eleventh Hour, I went along to this screening in LA and journalists put their hands up, and one of the first questions was, "What will happen when he reaches the thirteenth regeneration?" There's a fascinating academic study to be made out of how some facts stick and some don't – how Jon Pertwee's Doctor could say he was thousands of years old, and no-one listens to that, and yet someone once says he’s only got thirteen lives, and it becomes lore. It's really interesting, I think. That's why I’m quite serious that that 507 thing won't stick, because the 13 is too deeply ingrained in the public consciousness. But how? How did that get there?"

- Russell T Davies