; Date: April 9, 2017
The official announcement came courtesy the BBC: http://www.radiotimes.com/news/2017-04-06/john-simm-is-returning-to-doctor-who-as-the-master and http://www.bbc.co.uk/newsbeat/article/39513437/john-simm-is-returning-as-the-master-in-doctor-who
“I can confirm that it’s true, thanks to the power of time travel I’m back,” Simm said. “It’s always a pleasure to work with this great team of people and I can’t wait for you all to see what The Master gets up to in the next series.” Steven Moffat, writer and executive producer, added: "Nothing stays secret for long on Doctor Who but you'll have to wait a little bit longer to see exactly what The Master is up to and how he makes his return to face the Doctor. “It’s been a huge pleasure to have fan favourites John Simm and Michelle Gomez face to face in the same role! It’s not often you get to see a solo personality clash."
In one of the videos below, we see Stephen Moffat being asked in 2015 about the potential possibility of maybe someday would John Simms ever come back. He answered that was possible and then exclaimed about the craziness that would ensue if John Simms's Master and Michelle Gomez's Missy would play opposite each other. Clearly he's been thinking this over for several years now.
How? The Doctor has had several instances of multiple incarnations of himself meeting at the same place/time to work together. Even though that's supposed to violate some kind of causality whatever stuff, it's happened multiple times. Why would The Doctor be the only Time Lord for whom this can occur? Why not The Master?
Will Master and Missy get along? Usually the Doctor's instances don't get along when they meet. It's possibly Master/Missy will also not get along, right?
The Director of the episodes in question, Rachel Talaly, wrote on Twitter: "Be Afraid" https://twitter.com/rtalalay/status/850021861119455234
When in The Master's timestream? Someone at The Radio Times has more time than I, and says John Simms' Master is The Sixth Master ( http://www.radiotimes.com/news/2017-04-06/how-will-john-simms-master-return-to-doctor-who), making Missy the seventh incarnation. However one of the videos below shows the Roger Delgado Master was his 13th body, hence every time we've seen The Master since involved The Master stealing someones body. Since the showdown with Rassilon was probably the end for that incarnation, the instance of John Simms' Master has to come from a time prior to The End of Time.
Utopia showed us the beginning of the Sixth Master incarnation. But there isn't there direct continuity between the end of that episode and the beginning of The End of Time? At the end of The Last of the Time Lords the Master was shown to have died convincingly, including his body being burned by The Doctor, and an enigmatic hand picking up his ring. That ring was later used in a ceremony during which he was resurrected by a "cult" loyal to The Master, until his ex-wife Lucy Saxon sabotaged the ceremony. The crazy condition he had during The End of Time was due to the sabotaged ceremony.
In other words, there's no convenient time between the beginning of The Masters sixth incarnation and its supposed end for a meeting with Missy to occur. Unless there's ... well we'll just have to see what Moffat and crew come up with.
For example, between Utopia and when the Doctor and crew arrive on the scene in The Sound of Drums, The Master had already had several months to set up shop on his latest scheme. Then between Drums and The Last of the Time Lords there was a year where The Doctor was kept in his cage while Martha Jones saved the world. It's between The Masters ending and The End of Time where it doesn't appear possible for The Master to be available to meet with Missy.
Another point to consider is that at The End of Time we don't see The Master's actual death. We see him fighting Rassilon and other Time Lords. What if he actually survived in which case he might have made a deal with Rassilon? It could be used to explain how Missy was able to come into existence, because Rassilon would otherwise have been able to eradicate The Master from the universe.
Why? When the Doctor has multiple incarnations meet there's a higher purpose to that. That the issue being faced requires more oomph than just one Doctor. So, does that mean the mischief and mayhem to create requires more oomph than a single Master?