Absolutely amazing episode. Good mystery, clever answer, and it was creepy. Don't even know where to start with the praise for this one... It's nice to see the doctor without the sonic or the tardis, I don't remember any episodes playing around with alien languages so it was cool to see them without the translation matrix.

Also I know some people are gonna be annoyed that the timeless child stuff wasn't retconned, but honestly I'm happy with with they're doing with it. Chibnall had ideas but didn't do a good job of actually making it effect characters. For example, during Flux a few companions were stuck in the past for like 5 years and basically nothing changed about them - no trauma, no growth, nothing. RTD using Flux as a modern "time war" of sorts is exactly what I wanted from it in the first place - it's something that gives the doctor immense guilt and we actually see that in this episode. I totally think that Chibnall's era was horrible but if RTD can make lemonade out of lemons, I'm all for it.

But like, man that was a good premise. Good mystery, amazing setup - that moment when the doppelgangers are revealed was so well done, I thought it was just weird editing until I realized what was going on. Also the two parallel confrontations, knowing that one person in each is fake, but not knowing who it is in either group... Then turning the doctor's need to figure things out against him... Good shit.