"Extremis" - great episode! *spoilers*

This series of Doctor Who seems to be getting better-and-better in my opinion. In the latest episode, "Extremis," we finally learn what's in the vault. No surprise there, it's Missy, as many had already guessed.

There are two storylines: One in the past, and one in the present. The narrative jumps between them through out the episode. In the flashback, we learn how Missy came to be in the vault. In the present, The Doctor, while still blind and trying to hide it - he wears a pair of souped-up sunglasses that feeds him information about his surroundings - gets an inconvenient visit from The Pope! It seems there's this book that causes anyone who reads it to kill themselves. Only The Doctor will be able to read The Darkhold, err... The Veritas and survive. Of course The Doctor can't actually read it because he's blind, which no one but Nardole knows. The Doctor accepts nonetheless.

The Doctor, Bill, and Nardole journey to a secret library underneath the Vatican, where they meet a missing translator. Of all the people who'd read The Veritas, only this man hadn't killed himself. Why? Because he wanted to wait until after he'd completed emailing his translation to different groups around the world, including CERN. CERN's reply: offers of prayer!

What has everyone who reads The Veritas so afraid? Nardole, Bill and The Doctor eventually learn the truth: They are in a computer simulation of Earth created by a race of aliens as practice for their eventual invasion. Nothing is real. Not the Pope, not the Vatican, not CERN, not Nardole, Bill, or even The Doctor!

This is the first of a three-episode story arc. We don't know very much about the creepy alien race, such as how they plan on taking over Earth, but we do know The Doctor simulation has sent his real self a warning about the aliens. We'll see how it plays out in the next two episodes.