'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.
The Eleventh Doctor (Matt Smith)'s regeneration broke with this trend by being much briefer than the others, although the general effect was the same. The aged Eleventh Doctor is shown to regress back to his (normal) younger age before changing, after having been given a new set of regenerations; it is suggested that this process is the reason for these differences.
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), the Master regenerates into the female "Missy" (first appearing in Deep Breath), and a Time Lord general regenerates on-screen from a white man into a black woman in Hell Bent (demonstrating that Time Lords can change race as well as gender).
In the classic series it was established that a Time Lord can regenerate only twelve times. This limit remains in the new series, although for some time it was ambiguous as to whether it was still the case. With the War Doctor, and the Tenth Doctor regenerating twice (Journey's End), Matt Smith's Eleventh Doctor actually counts as the thirteenth incarnation; this limit was resolved in his swan song episode Time Of the Doctor where the Time Lords give The Doctor a new set of regenerations (how many is unclear).
It has been established that certain things can "fully" kill a Time Lord and prevent regeneration. 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 Doctor Falls, The Master kills his future incarnation (Missy) with his Laser Screwdriver, commenting "don't bother trying to regenerate, you got the full blast".
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 four of his 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".
At the very least, there appears to be an unconscious element to how new bodies are "chosen": in The Girl Who Dies it is established that the Twelfth Doctor unconsciously chose the face of Caecilius from the Tenth Doctor story The Fires of Pompeii as a reminder to himself that although he cannot change historical events, he can still help people.
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.
Time Lords appear to be able to consciously "hold back" from regenerating, or to temporarily delay the process if they choose. The Tenth Doctor does this before regenerating, as does the Twelfth Doctor in The Doctor Falls, thrusting his hands into a snow bank and announcing he will not change. In The Last of the Time Lords, the Master, mortally wounded, refuses to regenerate in order to spite the Doctor.
Another comment by the Doctor in Nightmare in Silver suggests that he is at least capable of intentionally triggering the process: "I could regenerate now. Big blast of regeneration energy; burn out any cyber widgets in my brain". However, as the Eleventh Doctor was (at the time) the last possible incarnation, it is likely that he was bluffing about having been able to do this.
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. The Eleventh Doctor repeats this in Time of the Doctor, where he destroys various Dalek attack ships using long beams of regeneration energy that extend from his arms.
- Day of the Moon (2011) - Young Girl (River Song)
Cause: Shot by Amy Pond. It is later revealed (in Let's Kill Hitler) that the girl was River Song, and that she regenerated into a toddler on the streets of New York.
- Let's Kill Hitler (2011) - River Song
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.
- Hell Bent (2015) - Time Lord general
Cause: Shot by The Doctor. Also notable for being the first on-screen depiction of a Time Lord changing both gender and race, as the general's new incarnation is a black woman.
- 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.