Oh, gosh. Sometimes this show makes it very difficult for me to pick a score out of ten. I liked watching this episode quite a bit, but my brain kept interjecting with complaints all the same. Let's take a closer look.
Cons:
In brief, this plot thread tells us the origins of one of the Doctor's most fearsome foes: The Cybermen. Cool. It does so with a twist about a black hole, and time moving more swiftly at one end of a large ship than it does the other. Also cool. Then there's the additional twist that the kindly yet strange old man who has been caring for Bill during most of the episode is actually the Master. C...c-cool? I mean, seeing John Simm back in that role is absolutely thrilling, don't get me wrong. And the idea of watching Missy and the Master riff off of each other is just delightful. I suppose my problem with this is that it seems like a twist for the sake of a twist. You could say that this older, even more evil version of the Master showing up is a chance for Missy to decide where her allegiances lie, and all of that. But for my money, Missy is plenty interesting on her own. There's so much to untangle with her character, and now that we're adding Simm to the mix, I worry that Missy's development will be halted or ignored. This is the first episode where Missy has had anything to do beyond an extended cameo, basically, and she's being overshadowed by the return of a sinister face from the past. Just a little bit hard to swallow.
I rather loved the conceit of Bill being separated from Nardole, the Doctor, and Missy, even though we just saw that happen in last week's episode. But I couldn't help but notice that in last week's episode, Bill had a lot of agency and tried to figure out what was going on. In this episode, however, Bill is taken in by the Master (albeit unbeknownst to her and us) and she sits around waiting for the Doctor to rescue her. Literally. It's a significant plot point. Bill hasn't suddenly become "the girl who waited" or the "mystery girl" or any of the other pithy and sexist epithets given to Moffat's companions in the past. But Bill starts off this episode getting shot, and ends it crying to the Doctor that she waited for him, apparently turned in to a Cyberman. I'm not going to rant and rave and get all angry about this, because I'm waiting for the finale to see how this all shakes out. But if things stand as they are, it's a little annoying, is it not? Bill sits around and waits, and then she ends up being one of the Doctor's most fearsome foes. Sound familiar? Did we not meet Clara originally in the form of a Dalek, and then see her again as a Dalek in a later episode? And now Bill has succumbed to a strangely similar fate.