Where does Doctor Who's latest Master fit into the character's long history? The Doctor Who season 12 premiere sprung an almighty surprise with the timely return of the Master. Sacha Dhawan spent almost the entire first episode posing as a secret agent called O, pretending to help the Doctor investigate a series of alien attacks on Earth's spy networks, but the final scene revealed Dhawan was actually the Master. The dastardly Time Lord was behind the entire plot, looking to wipe out humanity and prove his superiority over the Doctor once and for all.
Traditionally, the various incarnations of the Master have been presented in regeneration order, much like the Doctor. Roger Delgado started the sequence, and the villain goes through a protracted death before ultimately stealing the body of Anthony Ainley. Some time later, the Master is captured and "executed" by the Daleks before snaking his way into the form of Eric Roberts. In the new Doctor Who series, the mantle has passed from Derek Jacobi to John Simm, and then to Michelle Gomez, who took the title of Missy. The last time fans saw Missy, however, she had been left for dead by her predecessor, who insisted she wouldn't be able to regenerate.
Missy's fate is left on a cliffhanger until the Sacha Dhawan reveal and while it's standard practice for the Master to reappear without explanation, the new version does beg the question of how Missy managed to regenerate. Except, maybe she didn't. Throughout Dhawan's first episodes as the Master, there's nothing concrete to suggest that he is the next chronological Master, and it's perhaps more likely that this regeneration actually comes far earlier in the line.
Since the new Master mentions the Fourth Doctor's regeneration scene in "Spyfall Part 2," Dhawan has to be playing a post-Ainley regeneration. This means that Dhawan's Master can realistically only fit into two places on the Master's known timeline: the gap between Ainley and the Doctor Who TV movie's Eric Roberts, or the period after the movie, between the Master's imprisonment in the Eye of Harmony and his reemergence as Professor Yana in new-Who. In both cases, there are a multitude of novels and audio stories set within these gaps, but even though The Night of the Doctor made the audio series canon, the main TV show isn't bound by material featured elsewhere.
Several moments in "Spyfall" suggest Sacha Dhawan is playing a post-Ainley Master. He's certainly far more villainous than Missy and harbors the same wretched desire to bring down the Doctor as his older iterations, with motivations and a menacing demeanor more in common with the Masters of yore. The "new" Master also revives elements from the classic series such as the Tissue Compression Eliminator, further linking him to the past.
Furthermore, O mentions meeting the Doctor as a man and implanting himself within MI6 years before the events of "Spyfall." Although Doctor Who's time travel ensures nothing can ever be definitively ruled out, this line suggests Dhawan's Master has been alive for quite some time waiting for his moment, whereas a Missy regeneration would surely jump straight into action. Lastly, slotting Dhawan one incarnation before or after Eric Roberts not only helps plug a gap in Doctor Who lore, but also avoids the need to explain how Missy survived.
With that said, there are several key factors pointing to Dhawan's Master being a direct successor of Missy. Chiefly, the Master reveals that he found out Gallifrey's game-changing Timeless Child secret and wiped out the planet entirely in retaliation. If Dhawan, as an older Master, had discovered this shocking truth, John Simm and Michelle Gomez should've already known about it and told the version of the Doctor they encountered. Unless some memory wiping comes into play, this could create a paradox in the Master's timeline.
Fortunately, Doctor Who showrunner, Chris Chibnall, has already addressed all of the kinks in the Master's timeline. In season 12's premiere, Sacha Dhawan, still posing as O, tells Graham about the Doctor's life, dropping the line, "a lot of inconsistencies, but it's very, very interesting." Similarly, the introduction of Dhawan himself and the ambiguity around his placement in the Master's line of regenerations not only creates further inconsistencies, but also makes things infinitely more interesting.
Doctor Who season 12 continues with "Orphan 55" January 12th on BBC and BBC America.