[Warning: Massive Spoilers About the Doctor Who Christmas Special Ahead]
Ever since Doctor Who showrunner and executive producer Steven Moffat let it slip that both John Hurt's "War Doctor" and the so-called "Meta-Crisis Doctor" counted as full regenerations - making Matt Smith's Doctor the last Last of the Time Lords - Whovians and other observers have obsessed over an algebraic equation: The Old Doctor's Death + X = The New Doctor's Life.
Going into last night's Christmas Special, we knew that we would get an answer to that question, but when we finally got to see more than Peter Capaldi's robust eyebrows in the TARDIS following Smith's explosive and emotionally eviscerating exit ("Raggedy Man, goodnight"), we were left with a few more questions.
Obviously, if you dashed past the spoiler warning, one assumes that you are aware that - much like the moment in the 50th Anniversary Special when the War Doctor's hand hovered over the big red button, perched on the verge of massacring two races to end the Time War and save the whole of space and time - the Doctor found himself in a situation where he could not get it right.
Here, the Doctor makes his last stand in a town called Christmas that sits on Trenzalore - the prophesized place of his death. The Doctor does this because he is without his TARDIS, but also out of an obligation to keep the people of Christmas and the people of the universe safe from the Time Lords who want to return through that pesky schism between time and space that has plagued this Doctor, and the consequences of their return - a new Time War.
Growing older and more feeble as the centuries crawl by, the Doctor explains to Clara - who has returned with the TARDIS after being lost in time - that his song is ending, clarifying the regeneration limit for her and us.
Obviously, this wasn't a surprise, but it does prompt questions like: how did the next Doctor (Capaldi) have a hand in the rescue of Gallifrey without the current Doctor noticing his presence? Also, doesn't this revelation undermine the scene in "Let's Kill Hitler" where the Doctor thinks that he will re-generate while talking to the TARDIS' voice interface (in the form of young Amelia Pond) after being poisoned by River Song's kiss?
In that moment, though, the TARDIS' voice interface tells the Doctor, "Regeneration disabled", so is it possible that the TARDIS is letting the Doctor know that he has hit his cap? That's just one theory out of dozens that will emerge following last night's episode.
How and why the Doctor got his new regenerations is a little less ambiguous. Clara, who was once again sent away, returns to be by the Doctor's side, but as he goes to battle the Dalek's one last time, she stays behind and begs the Time Lord's to help the Doctor, which they do, by sending him a new regeneration (or is it a new cycle of 12 regenerations? More questions.) through another gash in time and space.
The Time Lords ability to do this exists in the canon of Doctor Who (the Master was given extra regenerations), and though it isn't explicitly laid out, extending the same gift to the Doctor is surely in the Time Lord's best interests. The Doctor won't likely go back to Trenzalore to utter his true name into the schism to bring the Time Lord's back anytime soon (though, why wasn't Clara's chat enough to do it?), but he is the one who saved Gallifrey and he is the Time Lords' only hope for an eventual return.
The Doctor's recollection of his people has been hot and cold over the years, with a mix of reverence and disgust coloring his words, but while returning the Time Lords is no easy decision (or feat), and while they aren't exactly the Doctor's allies, he'll surely take the benefits of their desperate overture.
In the end, though, none of this really matters. That's not to minimize the importance of canon and the Doctor Who mythos, but some kind of regeneration reboot was guaranteed to keep the show on the air at a high point in its popularity. Whether you agree or disagree with how it was done, it's likely that you are excited by the future of Doctor Who (while mourning the loss of Matt Smith), but as is always the case, it's not the regeneration that matters, it's what the Doctor does with it.
Doctor Who is set to return in autumn 2014.