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In the year of it's 50th Anniversary, this Christmas sees another two momentous milestones for the program. The Christmas episode is the 800th full length episode (as originally transmitted) and the one in which we, hopefully, finally see the Doctor break free of his 12 regeneration limit. Now this may come as a bit of a shock to some of you, if you haven't seen all of the previous 799 episodes, or weren't really paying attention. He's the 11th Doctor isn't he? Well in name yes, but he's used up all his regenerations to get there.

So how do we know this to be true, and what exactly do we know about the regeneration process from the history of the show? Well there are a few key episodes that cement Matt Smith as the last Doctor of his normal cycle, but there's quite a bit more to consider across the history, to give you the full story of regenerations. Here's an episode blow by blow of the key clues we have been given along the way.

The Tenth Planet (1966). Up until this point our hero was just "The Doctor" but William Hartnell was suffering with ill health and beginning to struggle with the workload, so in a moment of scripting genius the concept of "Regeneration" was born and, as his body was wearing a bit thin, he changed into Patrick Troughton (Regeneration 1)

War Games (1969). The Doctor tells us "we can live forever, barring accidents", he just doesn't say how many accidents. Then he introduces us to "his people" the Timelords, who promptly force another regeneration onto him, turning him into Jon Pertwee (Regeneration 2). This is one of the contentious regenerations, and many argue that perhaps it didn't count, being forced by the Timelords, but the exact numbering of Doctors is confirmed much later.

In Spearhead from Space (1970). Having only just found out he's a Timelord, we also find out he has 2 hearts, so not only can he regenerate in the case of severe accidents, he has a back up heart to avoid them in the first place. But that doesn't help him in Planet Of The Spiders (1974) when over exposure to spider mental energy and blue Metabelis crystals turns Jon Pertwee to Tom Baker (Regeneration 3)

Tom Baker's reign gives us a lot more information about the subject, The Brain Of Morbius (1978) shows us images in the mental duel with Morbius, giving the impression Hartnell as not the first, this was indeed the intention of the writer, but never outright stated on screen. It also gives hints that the Elixir of Life, created by the sacred flame on Karn, is occasionally used by Timelords in some capacity. It's just as well the "extra Doctors" in Morbius were never confirmed because later that year (though in the next season), Robert Holmes reveals the 12 regeneration limit for the first time, as an explanation for the dishevelled appearance of the Master in The Deadly Assassin (1978).

Destiny of the Daleks (1979) sees Romana make an elective regeneration (she wasn't injured as far as we know), and is able to change her form selectively many times before settling on a particular look. We never really know why she was regenerating, or how she could control what she looked like, as the Doctor never could. Maybe it's a personal skill, maybe female Timelords are better at it, or maybe it's easier to control when not regenerating in a crisis (it certainly seemed less traumatic), but recent events in the Doctor's storyline drop hints that he will get better at it. One mystery we did have at the time was whether she was using up regenerations by changing bodies, but that was answered later in The Christmas Invasion.

Keeper of Traken (1981) shows the Master cheat the regeneration limit using the power of The Keeper to take over Tremas's body, so even without regenerations there is always a way out, but in the following story Logopolis it's the Doctor's turn again, falling from a radio telescope, saving the universe, Tom Baker changes to Peter Davison (Regeneration 4)

Mawdryn Undead (1983) Is a pivotal story for regeneration, it makes it very clear the limit is NOT a count of actual complete regenerations, but use of a a finite amount of energy "packets" contained within him (1 per regeneration), and after it is used he can no longer regenerate. He also establishes that he only has enough energy for 8 regenerations left, thereby confirming his as number 5 and ruling out the forced regeneration not counting (War Games) and earlier Doctors hinted at in Brain of Morbius. Later that year The Five Doctors establishes that a new set of regenerations can be implanted (the Time Lords offer a set to the Master), and the First Doctor (albeit played by Richard Hurndall) also confirms himself as the original. Eventually we come to The Caves Of Androzani (1984) and a nasty case of Spectrox Toxemia transforms Peter Davison to Colin Baker (Regeneration 5).

In Trial Of a Timelord (1986) we learn that there is "another Doctor" running around out there, known as the Valeyard. An amalgamation of his darker sides from somewhere between his 12th and final incarnation. We know he's after the Doctor's remaining lives, what we don't know is whether he is actually a future Doctor, or something that in some way manifested into physical form. But after famous last words of "Carrot Juice" the TARDIS is shot out of the sky in Time and the Rani (1987) seeing Colin Baker change to Sylvester McCoy (Regeneration 6). Though technically we see Sylvester McCoy change into Sylvester McCoy as Colin Baker wasn't around to film the sequence.

Some 3214 days after taking the role, Sylvester McCoy hands over to Paul McGann in the TV Movie (1996) after being shot in an alley (Regeneration 7), making him the actor to hold the role of "the current Doctor" for the longest, beating Paul McGann by a mere 18 days. The movie also established that the Eye of Harmony can be used to transfer regenerations between bodies, and that anaesthetic can delay or possibly destroy the regenerative process.

Skipping ahead for a little, to make sense of regeneration numbers, The Night Of The Doctor (2013) sees Paul McGann change to John Hurt (Regeneration 8 ). This also shows that the Elixir of Life can be used to steer a regeneration in a particular way. Though McGann fell 18 days short of the record for holding the role, because his regeneration out isn't shown until 2013 he holds the record for the longest time between regeneration in and regeneration out at a staggering 17.4 years.

One interesting note of the regeneration into Hurt is that they use an image of a very young Hurt, so you get the impression by the time you first see him in previously in The Name of the Doctor (2013) (yes tenses can be terrible with time travel) that he had worn this body for a very long time. By the time we see him for the third time in The Day Of The Doctor (2013), his body is also wearing a bit thing and we get John Hurt changing, we assume, to Christopher Eccleston as he was hoping for more conspicuous ears (Regeneration 9).

Back into transmissions sequence Rose (2005) is presumably shortly after events in The Day of the Doctor as we see the Doctor checking out those ears for the first time. He doesn't hold on to them for long though as in Parting Of The Ways (2005) he absorbs all the energy of the vortex, and nobody's supposed to do that. The Doctor tells Rose "you never know what you're going to get", implying that, at this point in his life, he can't control the outcome of a regeneration, and Christopher Eccleston becomes David Tennant, becoming alarmed at his new teeth (Regeneration 10).

In The Christmas Invasion (2005) we get new information that within 15 hours of a regeneration it is still possible to make tweaks and fixes, in the Doctor's case he is able to grow a new hand after having it chopped off by a sword, but this also explains Romana's multiple shifts.

Interestingly, in School Reunion (2006), we either have a bit of lazy script writing, or incredible foresight. When Sarah finally realises who the Doctor is, she says "You've regenerated", to which the Doctor replies "Half a dozen times since we last met". Now I'm sure the writer intended "last met" to mean "last travelled", with the Fourth Doctor, but they actually last met in The Five Doctors, and at the time that regeneration count just didn't add up. Now however, with the War Doctor... Who knew?

By 2007 in Utopia, we have it confirmed that indeed the Master DID get a new cycle of regenerations, as it shows Derek Jacobi regenerating into John Simm, and in Last Of The Timelords the Master shows that he can actually stop a regeneration from starting.

Journeys End (2008) shows David Tennant starting a regeneration after being shot by a Dalek, but then diverts the remaining energy into his severed hand, which eventually creates an almost complete Meta-crisis Doctor (minus 1 heart). The dialog clearly states he used as much energy as was needed to fix himself, and diverted the rest into a suitable receptacle (Regeneration 11).

"used the regeneration energy to heal myself, but soon as that was done I didn't need to change. I didn't want to, why would I, look at me? So to stop the energy going all the way I just syphoned off the rest into a handy bio matching receptacle, namely my hand"

This also shows the Doctor has gained some level of control over the process, being able to stop the change and divert the energy. But the key thing here is the dialog categorically states that all the energy was expended, enough to create a new body, so that's one less body for the Doctor. Curiously, despite being his biggest enemy, this is the only time in the history of the show that a Dalek gun has cost the Doctor a regeneration.

The End of Time (2010) sees the Doctor exposed to a lethal does of radiation, saving Wilf, but evidently it is a slow lethal dose as the Doctor has time to do a tour of all his companions before David Tennant changes into Matt Smith (Regeneration 12), and runs out of regenerations.

In Amy's Choice (2011), we encounter the Dreamlord, a projection of the darker side of the Doctor's personality. As we're now between his 12th and final incarnation could this be part of what is amalgamated into the Valeyard?

The Impossible Astronaut (2011) shows what appears to be Matt's Doctor starting a regeneration before being killed outright. This indicates that if a Timelord is fatally injured again DURING the crucial part of a regeneration they will die BUT this wasn't a real regeneration happening so this may not be true, and part of the overall ruse. This was the first hint to us that Matt cannot regenerate.

The Doctor's Wife (2011) confirms that The Corsair had achieved cross gender regenerations, but gives no indication whether this is a normal course of events or rare. Let's Kill Hitler (2011) shows Mels regeneration into River, during which she concentrates on a dress size, so either River can control appearance a little, or it's more of a female thing (see Romana), or she was "concentrating on a dress size" and didn't achieve it, since we never knew exactly what she was after. It also shows the extract of The Judas Tree can kill a Timelord and prevent the triggering of a regeneration (and a second hint that Matt cannot regenerate). It also shown that regeneration energy from one, can be used to fix another, which is then paid back a little in The Angels Take Manhattan (2012) which shows that Matt's Doctor still has a little regeneration energy left, as he fixes River's wrist.

Finally in The Day of the Doctor (2013) we also get a hint that the Doctor WILL go on, and in the future will have more control of how he looks - revisiting some old faces.

Hopefully that should catch everybody up on the history of regenerations and prepare you all for the coming Festive Feast - The Time of the Doctor.
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