Oh boo. New Who just got dropped by all major streaming services here without warning; I no longer have a place to watch legitimately. Simultaneously, a separate, free "service" for streaming shows that I may or may not have used has shut down. What a bad week for this to happen, I was very much looking forward to this one. I will try to figure something out so I can continue to join the rewatch.
EDIT: Just got done watching in about no-hundred-and-40p.
While I personally don't enjoy Moffat as a showrunner, he's excellent as a writer of episodic TV and that certainly shows here. My preference for the kind of low-budget jank Who is completely overridden by this episode, which has very high production values and is excellent. When I think of the Ninth Doctor, this two-parter is the first thing that comes to mind. It's the best kind of eerie and does a good job introducing all of the elements you'll need to understand even if you dropped headfirst into this episode.
It's funny how little genre-savvy Rose has here. A small child yelling "Muuuuuuuuummyyyyyyy" into the night is very rarely a good sign, but Rose is off like a rocket, trying to get as close to the scary nightmare child as possible, ASAP. But maybe kids calling for their mummies were less scary before this episode aired. Ultimately, though, it leads her into a story that's a different kind of creepy. Everyone else already said it, but her B-plot with Jack/Barrowman is the weakest part of the episode.
Barrowman's ... let's say "confidence", shines through in the character and doesn't acquit either very well. I'm probably a bad fan in that I never watched Torchwood, so all of my familiarity with Jack Harkness comes from his handful of Who episodes. Maybe I'd find him more likeable if I had more time with the character, but I don't feel like I ever warmed to him as much as the show hoped. It's not fair to compare anybody, anywhere to Harrison Ford, but as much as he might like to be, Jack Harkness is no Han Solo.
Getting back to the better part of the episode, Florence Hoath's Nancy is fantastic throughout. She's believably hardened by life in the blitz while her deeply sensitive nature beats along just below the surface. The episode is properly engrossing in all of her scenes and she's even able to carry some without any of our leads around. Surprisingly, she retired from acting around 2008; these days, she's a pre-school-targeted YouTuber, i.e. singing nursery rhymes and stuff.
The gas mask transformation is still incredibly effective. It's only a little bit wibbly-wobbly-2005y-wimey. Contrast it against Adam's head-hole a couple of weeks ago and this looks years ahead. It's aided by the capable performance of Richard Wilson as Dr. Constantine immediately prior. Great episode. If not for "Rose" already doing the job well, this would be a solid recommendation for new fans to see if they like what the show has to offer, because this remains some of the best.