Warning: SPOILERS for the Doctor Who season 10 finale.
What happened to Missy and The Master in the Doctor Who season 10 finale? 'The Doctor Falls' brought to a close the storylines for the vast majority of season 10's central characters; although the Doctor will wait to fully regenerate, this was Peter Capaldi's final episode before his big farewell in the Christmas special and, as we saw, his regeneration has already begun. It's goodbye, too, for Matt Lucas as Nardole, who has seemingly found a new life as some kind of foster carer who will spend the rest of his years shuffling children from one locale to the next in an effort to evade the Cybermen. Bill, it would appear, has now decided to embark on her own journey through space with Heather, and while her return is possible, it seems likely that with new showrunner Chris Chibnall incoming, we've also said goodbye to the excellent Pearl Mackie, as well.
Finally, the season 10 finale also saw the departure of Michelle Gomez as Missy, and John Simm as The Master. Though Simm's return was only brief, it was a joy to see him back on our screens again, and his chemistry with Gomez was sensational. So good, in fact, that it seems a shame that a spin-off is out of the question. Throughout Capaldi's reign in the TARDIS, Missy/The Master has been the continued antagonist for the Doctor; a thorn in his side that gleefully twisted and bore deeper throughout each season. That said, it's also undeniable that the Doctor had a fondness for Missy, and a quiet certainty that she would turn good in the end. In 'The Doctor Falls,' she very nearly proved him right.
Essentially one and the same person, The Master and Missy were never destined to live harmoniously side by side. While The Master has been absent, Missy's evil tendencies have been slowly ebbing away, until the Doctor contained her in the vault while she repented her evil ways for good. That vault seemed important at first but merely served as a way to bring Missy back into the center of proceedings. Gomez's performance, over the final two episodes of season 10, was certainly convincing; one never quite knew if she was really on the Doctor's side or not. Her ties to The Master were strong, and she almost felt compelled to do his bidding, but there was something more human about Missy, if you will - that element to her character that the Doctor also possesses, that gives her the ability to empathize with the plight of others.
Even so, there's no mistaking that Missy has, over the years, been accountable for a lot of wrongdoing, and however good her intentions, it's hard to trust that when someone like The Master is standing there, reminding us all that they are one and the same person. He is inherently evil and has no remorse. He doesn't care that others will die as a result of his actions, he doesn't care that humanity is at risk from the invasion of the Cybermen. In fact, he revels in it. It was that malice that eventually drove Missy to take the drastic decision to end his "life."
Being a Time Lord, The Master can't entirely die (at least not from a stab wound). Instead, will regenerate, like the Doctor. He's done that before, and the result was Missy. She knew that by killing The Master, it would force his regeneration, leaving her as the only Master. The problem? Missy chose to stand with the Doctor, too late. Two halves of the same whole, Missy is the positive while The Master is the negative. She obliterates him in order to save others, but The Master's hatred of anyone and everything knows no bounds, and his final act is one of self-destruction. Giving Missy the "full" blast with his sonic screwdriver, he has disabled her ability to regenerate, thereby bringing about the end of The Master once and for all. Or did it?