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.
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.