Ok, let’s speculate, how do you deal with two Masters (or a Missy and Master), Bill as a Cybermen and a huge spaceship full of Cybermen?
Simm Master says to Gomez Master ‘I’m concerned about my future’.
So Simm Master becomes Gomez Master at some point.
Okay, easy enough so far.
Therefore the Simm Master has to be killed, preferably by Missy.
To create a temporal paradox. Two Missies can’t be. If Simm Master regenerates into Gomez Master with Gomez Master already there.
Yes, it’s a giant Reset Button kids.
Time Lords from different points in their own existence should fizzle a bit, so we’re at Father’s Day in Series 1 with the Ninth Doctor a bit. Day of the Doctor has the excuse that Doctors Ten and Eleven were made to interact with the War Doctor through the influence of the Moment device.
But.. didn’t the Master die already with the Tenth Doctor in ‘Last of the Timelords’?
Kind of. Lucy Saxon brought him back for ‘The End of Time’. But same guy but blonder and madder and more well, everything.
Whether the events of those two episodes are even relevant I don’t know, and is Simm-Beardy-Master before or after Simm-Blondie-Master in timing? The reference to Harold Saxon means he’s definetly after Simm-Saxon-Master, but we know nothing else.
If it’s a linear progression of Simm-Saxon-Master, Simm-Blondie-Master, Simm-Beardy-Master, Gomez-Missy-Master, then that’s simpler, but leaves the issues of two different incarnations of the same Time Lord at the same time and obviously their self interest not aligning vis-a-vis The Doctor. Simm-Master presumably hasn’t encountered the Twelfth Doctor before, but Gomez-Master owes him her life.
Okay, so there’s some dramatic tension between two different antagonists, but their whole relationship won’t be almost mutual automatic trust as between the Doctor’s different incarnations but, will have more nuance.
Simm-Master will realise that his existence can be ended resulting in the paradox. He’s created a paradox machine before from The Doctor’s Tardis. We have usually assumed Missy not to have a Tardis. But then again, how did she get off Skaro after the encounter with Davros in ‘The Magicians Apprentice/The Witch’s Familiar’
If Simm-Master isn’t the direct predecessor of Gomez-Master, then we’re off on the River Song sort of Timey-Wimey timeline.
If Missy-Master does die in The Doctor falls, then who does Missy then regenerate into? Presumably our Thirteenth Doctor needs a Master too. Obviously better if Missy’s fate isn’t resolved just yet.
So, logic leads me to Gomez-Missy-Master killing Simm-Beardy-Master but along the way is the question of The Doctor and Bill and at what point and level a paradox works especially in what is effectively a gravity well with time running faster at the part of the ship nearest the black hole.
Differential time, Differential effect is possibly the get out available to stop the finale merely being a Reset of events. Missy will well understand the effect of the Black Hole, possibly to know that killing the Master later might have less overall effect than earlier and lower in the ship.