Speculation is rife and ideas are plentiful, but one fan has a theory we can totally throw our lot in with.
When discussing whether or not Missy will remember the events of series 10 if she appears with The Master (only the current incarnations of Time Lords seem to remember their multi-Doctor adventures in full), Redditor AWildDorkAppeared suggested another possible plot thread for Simm and Gomez.
“When we met Missy, she mentioned the Doctor left him/her for dead. We assumed that to be an accusation on how Simm’s Master returned to Gallifrey during the Time War at the end of The End of Time” they wrote. “Perhaps it was instead referencing the Doctor leaving Simm’s Master for “dead” at the end of this adventure (because the Doctor knows he’ll survive), forcing him to regenerate into Missy.”
“An interesting retcon, but one of Moffat’s better ones”, said fellow redditor theReluctantHipster – we couldn’t have put it better ourselves.
That’s if it turns out to be true, obviously.
Simm himself seems to suggest that the Master’s comeback will almost certainly be accomplished by the series visiting the character at an earlier point in his timeline: “I can confirm that it’s true, thanks to the power of time travel I’m back,” he said.
Could it be that we’ll see him left for dead by the 12th Doctor, only to regenerate into the Time Lady we’ve come to know and love to hate? It still wouldn’t explain why they appear on screen together, but it could tie up some loose ends rather nicely.
We’ll just have to give it space and time…
Doctor Who continues on BBC1 on Saturday nights at 7.20pm