Michelle Gomez's turn as Missy has been a joy over the last few series of Doctor Who, and having her former self – John Simm's Master – on screen at the same time in the most recent two episodes was a rare treat: the first multi-Master episode ever. But it seems the Master is such an egotist that he can't bear sharing the screen for long, even with him/herself.
In a bluff that perhaps even she wasn't completely sure about until it happened, the newly repentant Missy decided it was the Doctor, not the Master, she would stand with, and during an embrace with her other half, slipped a sneaky blade between his ribs.
"Now that, was really, very nicely done," said an appreciative Master.
But not to be outdone, the Master turned the deadly beam of his laser screwdriver on Missy, telling her "Don't bother trying to regenerate, you got the full blast."
If the Master really has killed Missy – ie damaged her so much that he's prevented her from regenerating – then this could be the end of the line for the Time Lady, just when things were getting good. Maybe that makes sense – after all, what use is an arch-villain without the evil?
But is that really the last we'll see of Missy/the Master?
Michelle Gomez has been pretty clear that this is it for her Doctor Who career. “It’s done now. It’s over. It’s the end of a chapter,” she said. And incoming showrunner Chris Chibnall will certainly want to put his own mark on Doctor Who – he is, after all, starting with a new Doctor and a new companion – so he's unlikely to want to bring back Steven Moffat's creation any time soon.
But what about other incarnations of the Master?
Simm's Master claims to have done Missy in for good, thereby extinguishing her future regenerations, but throughout the episode it seemed as if she had been one step ahead of him. Was that smile on her lips as she apparently prepared to die simply one of appreciation of the irony of the situation or did she know something the Master didn't? Does she have one last card up her sleeve that might mean the Master hasn't wounded her quite as mortally as he thinks he has – the equivalent of a bullet-proof vest?
Or could she find new regeneration energy from somewhere else? Perhaps the Time Lord council, realising that she has turned over a new leaf (or maybe because they have another use for her) might grant her a new regeneration cycle like they recently did for the Doctor, and have done for the Master in the past.
Remember, too, that John Simm's Master isn't dead – he's about to regenerate into Missy (we assume), back when she was still bad. So if she then decides to pursue the Doctor it would be in her evil form. Yes, the existing time line says that will involve an earlier failed attempt to beat the Doctor with a Cyberman army (see series eight finale Dark Water/Death in Heaven) but perhaps, having hung out with her older self, she's now armed with information that will prompt her to change her plans and go after the Doctor a different way.
But wait – both Missy and the Master seem a little unclear about the whole who-regenerated-into-who-and-when thing anyway...
"So I imagine you're the next one along then?" asks the Master as they sashay around a rooftop at the start of the episode.
"Er, I think so," says Missy. "I'm a bit hazy on the whole regeneration thing I'm afraid."
We never actually see the Master's regeneration after he's stabbed, so perhaps there are other incarnations to come, after him but before Missy. It wouldn't be the first time Steven Moffat has sprung a previously unknown version of a Time Lord on us, and perhaps this is his parting gift to Chris Chibnall who will no doubt one day want to cast his own version of the Master.
Finally, let's not forget, this is Doctor Who, a show about a man with a time machine. It would be a shame (and seems unlikely) to think we'll never see a new version of the Master again but even if that were true, time travel means there are always new adventures to be had, even if they happen in the past.
One way or another, it seems unlikely we've seen the last of the Master. And that can only be a good thing...
Doctor Who returns to BBC1 for the Christmas special