Bill claims to know more about the disappearance of the Ninth Roman Legion and almost proves herself right. If not for the inter-dimensional light eating monster, she might have been. Nevertheless, she gives the Doctor a run for his money in all things. She is intelligent and brave and charming and self-sacrificing. She’d make a good Time Lord herself.
The episode has great elements to it. We have Romans who are always welcome, Scotland which is awesome, a dimensional rift and a fairly terrifying monster. The Doctor, of course, walks into the maximum danger with the confidence that things will sort themselves out.
As Bill considers her experiences in the TARDIS and understanding of the Doctor we get a better understanding of him as well. Not just for Capaldi’s version, but rather with an overall sense of perspective on them all. It is easy to see how, to this nearly immortal and all-knowing god-like alien, everybody just sounds like children.
Bill’s assumption of the leadership of Legion survivors is compelling too. Her line “you won’t all die in a hole in the ground” is so Doctor-ish. She’s learned so much and mostly for good. I really hope she is around for next season.