Extremis
Extremis to this whole thing is that the Doctor is blind. Now, I cannot say that is why the episode is called Extremis. I really can’t wait to see what is really going on. From the trailer is look as if they are going back to earth. The is now lying to Bill. He does not tell her he can’t see. That is making of a good drama.
Personally if I am the Doctor, I do the same thing. Nardol knows that the Doctor cannot see, but does not understand why the Doctor will not tell Bill. My thoughts on this is that Bill does look up to the Doctor. With that being said, he does not want her to see him weak. Now, being blind does not make you weak , but it does put you in the dark literally. You do not know where you are going and if your are not use to it it can lead to problems. He of course does not wish for any of his enemies to know this. That can cause serious problems.
As I watch the trailer, what I see is that they seem to be in a Catholic Church maybe. There is a man I am supposing guarding documents of some sort. There is a secret and the TARDIS takes the Doctor there to help deal with a specific problem. What that problem is I do not know, but rest assured the Doctor will find it and do his best to protect the people and resolve the issue.
Another intriguing is somewhere in the episode he seems to ask advice of whoever is in the vault. Then we get a glimpse of Missy. Wow, now that is wild. Is it a spoiler or just a tease. Who knows but Steven Moffat and the Doctor crew and cast. I can’t wait to see what happens this Saturday at 8 est/7 cst.