's excellent writeup gives a detailed description of regeneration from classic Doctor Who
; however, it was written before the start of the revived series of Doctor Who (which began in 2005). In the new series, the concept of regeneration, while retaining the same purpose of allowing new actors to take over the role of the Doctor, has been extended in various different ways.
One change is consistency to the regeneration effect. In the classic series, a variety of different effects were used to transition from one Doctor to the next. The production team for the new series instead decided that there should be a common regeneration effect. The effect takes the form of a fire that engulfs the Time Lord's body, consuming the old body and transforming them into their new form. In The End of Time, the Tenth Doctor (David Tennant)'s regeneration is shown to be particularly violent, with this "fire" from his regeneration process causing the TARDIS control room itself to catch on fire.
There are recurring mentions of "regeneration energy" throughout the series; the Doctor is observed to exhale a yellow cloud of this energy after regenerating into his Tenth and Eleventh incarnations (Seen in the Children in Need special, The Christmas Invasion and The Eleventh Hour). In The Christmas Invasion, the Doctor, recovering from his regeneration, comes under attack from alien "pilot fish" who have detected his regeneration energy and want to use him as a power source.
Regeneration continues to be a traumatic process for the Doctor to undergo. In the Children in Need special, he is observed to behave erratically, commenting that "the regeneration is going wrong" and that he "can't stop himself". He subsequently spends much of The Christmas Invasion in bed, eventually being revived by a cup of tea that spills into the TARDIS. In The Eleventh Hour he is also shown to experience occasional moments of physical pain, expressing annoyance at having to fend off an alien invasion as he is "not done yet".
The Eleventh Doctor story The Impossible Astronaut shows that a Time Lord can be killed (properly) when he is in the process of regenerating. The Doctor is shot, causing his regeneration process to start, and then shot once more, which kills him. (Though this is later revealed to be the Teselecta, a time ship impersonating the Doctor, and not actually the Doctor).
More has been revealed about regeneration through the character River Song (specifically in the episode Let's Kill Hitler), who has at least a partial Time Lord-style physiology as a result of being conceived inside the TARDIS. The ability of Time Lords to regenerate is explained as resulting from their exposure to the Time Vortex. River kisses the Doctor with a lipstick containing a poison extracted from the Judas tree that prevents regeneration.
Time Lords are able to voluntarily give up their regeneration energy to heal others. The most notable example of this is in Let's Kill Hitler where River Song gives up her remaining regenerations to save the Doctor from dying. The Doctor later partially reciprocates in The Angels take Manhattan by giving up some of his own regeneration energy to heal River's wrist, an action for which she chides him as being a waste.
It has also been established that a Time Lord can change gender when regenerating, as the character The Corsair is described as having been both male and female (The Doctor's Wife).
It remains unclear what degree of control Time Lords have over their regeneration. In Utopia the Master
states that he will become "young and strong" like the Doctor, before regenerating into a new incarnation played by John Simm
. However, in the 2005 Children in Need
special, the Doctor specifically states that he cannot "change back" into his previous incarnation, and all three of the new incarnations have shown unfamiliarity with their new appearance after regenerating.
In Let's Kill Hitler, River Song claims to be trying to "focus on a dress size" before regenerating, implying that some degree of control might be possible. Night of the Doctor shows that the Sisters of Karn have "elevated Time Lord science" that allows them to control regeneration with elixirs. In Day of the Doctor the Doctor encounters The Curator, who is strongly implied to be a future, retired version of the Doctor (played by Tom Baker) and claims to be "revisiting old faces".
It also remains ambiguous whether the regeneration process requires conscious effort on the part of the Time Lord to initiate, or if it happens automatically. In The End of Time, the Tenth Doctor absorbs a lethal dose of radiation, and the regeneration process appears to initiate immediately afterwards, as he is able to simply "wipe away" a cut from his forehead using his hand. After saying goodbye to his former companions, he comments, "I don't want to go", before being overcome by the full regeneration process. However, in The Last of the Time Lords, the Master, mortally wounded, refuses to regenerate in order to spite the Doctor. In Turn Left, the Doctor is seen to have been killed in an alternate timeline, with speculation that things happened "too quickly" for him to regenerate.
In the classic series it was established that a Time Lord can regenerate only twelve times. It isn't clear that this is still the case in the new series. In the Sarah Jane Adventures episode Death of the Doctor, the Eleventh Doctor says that he can regenerate "507" times, although series producer Russell T Davies explained in an interview that he was merely being facetious. The BBC Series 4 FAQ suggests that there is no longer a limit, now that the Time Lords are gone.
As the series has now reached the eleventh incarnation of the Doctor, the BBC obviously wish to remove the limit, although it remains to be seen if the issue will be explicitly dealt with before the thirteenth regeneration is reached. There is some speculation amongst fans that this may have already happened when River Song gave up her remaining regenerations in Let's Kill Hitler.
The regeneration concept has been used for numerous plot devices. In The Christmas Invasion
, the Doctor loses his hand in a sword fight with the Sycorax leader, but miraculously grows a replacement hand, explaining that he is able to use the remaining energy from his regeneration to do this, as he is still within the first 36 hours of his regeneration cycle.
The Doctor's hand becomes a recurring plot device in its own right, being used by Captain Jack to detect the presence of the Doctor, among other things. In Journey's End, the Doctor is able to avoid regenerating (after being shot by a Dalek), by diverting his regeneration energy into the hand. There follows an extra plot twist as the hand is then able to grow into a half-human clone of the Doctor with the help of Donna Noble, an event referred to as a "meta-crisis".
Similarly, in Let's Kill Hitler, River Song is shot by a group of Nazi soldiers, but survives due to having recently regenerated. She produces a wave of regeneration energy that not only heals her, but also knocks out the guards.
- Let's Kill Hitler
Cause: Shot by Hitler. Also a notable episode for introducing several other regeneration concepts: the Judas tree, the ability of Time Lords to willingly give up regenerations, and the ability to wield regeneration energy as a weapon.
- http://www.bbc.co.uk/doctorwho/s4/faq/plot_continuity.shtml - BBC Series 4 FAQ
- http://www.sfx.co.uk/2010/10/26/interview-russell-t-davies-talks-about-that-sarah-jane-adventures-line/ - Russell T Davies interview with comment about regeneration limit.