The season 10 finale of Doctor Who contained a number of integral moments, but perhaps none more so that the final two scenes, and the return of the first Time Lord. Confirming what was already rumored, recent set photos released show David Bradley as the first Doctor, originally played by the late William Hartnell. After a fierce battle with the Mondasian Cybermen, and fulfilling his vow that he will fall where he stands, the Doctor fell victim to viscous blasts from a cyberman, and was rendered unconscious.

What would normally kill a human, of course, does not have the same effect on the Doctor. Fans well know that Peter Capaldi will be leaving Doctor Who at Christmas, and the time has now come for the twelfth Doctor to regenerate. Showrunner Steven Moffat, who is also leaving, has been hinting at a 'different' type of regeneration throughout the season, and there's been a continued sense of foreboding throughout as if the Doctor has somehow known this regeneration is imminent. There's a good chance he has indeed known for some time, and that could well be why, in 'The Doctor Falls,' he was seen desperately trying to stop it.

Peter Capaldi in Doctor Who Season 10 Episode 11

As things stand, the Doctor has lost Bill; someone who had shared his spirit of adventure and gave the Doctor a chance to see time and space with a fresh pair of eyes. At the end of season 9, the Doctor was tired and jaded. With the arrival of Bill came a new lease on life, and he began enjoying his travels again - so much so that he repeatedly went 'offworld' when he was supposed to be guarding Missy in the vault. He wanted Bill to experience all that being the Doctor's companion had to offer, and she lapped it all up. Their friendship was a joy to behold, and that is why he tried so hard to save her when she was turned into a cyberman. But it might well be that Bill has ended up saving the Doctor.

Following rumors that Pearl Mackie won't be staying under incoming showrunnner Chris Chibnall's leadership, it certainly looked as though Bill said her final goodbye to the Doctor at the end of the finale. Heather reappeared, and the two took off to explore the galaxy together, with Bill seemingly thinking the Doctor had died. As she kissed him goodbye, she shed a tear, which revived him and seemed to start, or re-start, perhaps, the regeneration process. Back in episode one of season 10, 'The Pilot,' when Heather had to leave, Bill cried and said the tears weren't hers, meaning they belonged to Heather. Did she save her own tears for the Doctor? Do the tears she now cries have some form of healing power?

It's unknown if Mackie will be back on a regular basis, but with Heather stating she could return her to her human form if she wanted, the door certainly remains open for her return. It's very likely we will see Bill back in the Christmas special, but that could merely be in flashback form. The rest of the cast have yet to be announced for the Christmas episode, but it's said to be mainly a two-hander, between Capaldi and Bradley, and that's what makes it, and the impending regeneration, so exciting. Bradley has played the first Doctor before, or at least the actor who portrayed him in 2013's “An Adventure in Space and Time,” when he took on the role of William Hartnell, which he executed brilliantly. It's an exciting prospect to have him back on our screens, and sharing it with another Doctor who is also renowned for his quirky, slightly curmudgeonly style.

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Missy and John Simm's Master back to back, with the 12th Doctor between them

At the end of 'The Doctor Falls,' the Doctor, revivied by Bill's tears, emerged from the TARDIS in the midst of regeneration, vowing not to live on anymore. Did the Doctor want to, or was he at least willing, to die? Or was he merely wanting to stay in his current form to save Bill? Either way, he's not in a great state of mind after seeing so many lives lost, plus Nardole having to leave his side, and Bill stuck as a cyberman. He's immediately told, by a figure emerging from the mist, that this desire to stop the regeneration is a ridiculous notion. It's a shame, in a way, that Bradley's return had been leaked, because his arrival could have been a great moment, but he is back anyway, and on fine form as a complete Hartnell doppelganger. We will see a lot more of him in the Christmas special, and Moffat says we will see his TARDIS, too, which is also an exciting prospect, but why is he back?

On the surface, it appears that the first Doctor, "the original, some might say," is here merely to convince the Twelfth Doctor that it is time to regenerate and that he must allow the process to take place. The synopsis doesn't give anything away in that regard, but could it be possible that 12 will initially regenerate back into 1, before becoming 13? That would be a long, and a way of going about things, and a far better option would just be for the first Doctor to point out all the reasons 12 has to go through with the regeneration- not that he can halt it anyway. That would certainly deliver on the emotional front, given that Capaldi's Doctor has been through a lot, especially with Clara Oswald.

Moffat has teased a very emotional Christmas special, and he's written all of the episode bar the scenes with the new Doctor, which Chibnall has penned. While that is the moment fans have been waiting for, there's still a lot to come before that, including a rumored reappearance from Jenna Coleman as Clara Oswald. It's become tradition for the Doctor's companions to return for a farewell at regeneration time; Billie Piper came back as Rose for a heartbreaking farewell to David Tennant's tenth Doctor, and Karen Gillan was back to say goodbye to Matt Smith. It follows, then, that Coleman would return in some capacity, however brief, after sharing so much with the Doctor. That could be a real tear-jerking moment; the twelfth Doctor had all his memories of Clara wiped, so whether he knows who she is at all is one issue, and if he does, then it's bound to be a bittersweet reunion.

Then there's Nardole and Bill; surely if Clara gets a goodbye of some sort then they should too? So that's three potential farewells, plus the original Doctor guiding the twelfth through his regeneration. Now the final question remains; who will replace Capaldi? The thought of it being Bradley is far-fetched, but it would be fun, even if only temporarily. Some are theorizing that Bill's tears triggering the regeneration process point toward the next Doctor being female; again this seems an outside bet at the very best. Meanwhile, betting has been suspended on Kris Marshall, who was hotly tipped to take over the TARDIS controls. In a society where nearly everything is spoiled or leaked beforehand, it would be nice if, just once, Doctor Who had a complete surprise in store.

Doctor Who will return to BBC America for Christmas 2017.

Next: Doctor Who Season 11 May Just Have A Single Storyline