"Memories are so much worse in the dark."
So it's Missy in the vault, but this raises so many more questions. Why was she being executed? Why did the Doctor agree? How did she get captured? Who is holding him to the oath? etc. I mean, some of it could be a sense of duty. We did see him go through that dilemma at the end of Peter Capaldi's first season, and decide to pull the trigger on her then. But now he can't, so something has changed. He's lost more people now. Clara. River. He's stuck on Earth but adrift. (btw, some are saying Missy's been in the vault only 70 years or so, but actually we have no idea how long it's been; we only know that the Doctor has been at the University with the vault that long).
He has always felt a responsibility for the Master. One can certainly see him feeling the obligation to make sure Missy doesn't escape after he doesn't actually kill her, so the oath he's keeping may just be his own.
What's interesting though, is this turnaround from how Nardole sees the vault and its occupant, and how the Doctor does. Because the Doctor doesn't want anyone to know he's blind (this apparently includes both friends and enemies), but the first person he tells is Missy. Unless she can't hear him through the vault but it seems from "Knock Knock" that she can. Last episode Nardole was berating the Doctor for possibly getting sick or injured and reminding him that "their friend" down there would know, and presumably would prey on that weakness. The Doctor doesn't seem to have any misgivings about telling her now. But then again, he is awfully, awfully alone at this point. Missy is the one link he has to his past, and he can't bear to not treat her like a friend no matter how inevitable the betrayal will be at the end.
So now we have the Doctor, who has stayed put for a pretty long time now, guarding the vault, already missing his old life, adventures, space, and already feeling conflicted about his choices and plagued by the memory of what he had to do (there is so much sadness in the words of the oath he says after he pulls the lever, it's astonishing), now blind on top of all of that and fighting the memories and regrets in the darkness. First 4.5 billion years of torment in order to save and then forget his best friend, now this. Twelve is not having an easy go of things.
So, that was a lot to say about the first minute or so of this episode, but it sets the scene for this particular crucible. Which as it turns out is just a dry run of the crucible for a simulation that is not the Doctor at all, except in every way it matters. So I have a few nitpicks about the episode--but overall the plot picks are far outweighed by the theme. I am always one to sacrifice plot for the sake of the theme; stories are much better that way than the reverse, and it is very difficult to make both work at the same time, especially if you're working in 55-minute television increments.
I will likely have more to say about it later--like how much I loved the scenes with the CERN scientist, how much I loved Peter Capaldi in the whole episode but especially the end, and a few plot nitpicks that I will bring up despite my willingness to hand wave them away. For now I'll just say that I really want to see next week's episode.