I remember feeling truly unsettled watching this for the first time. For one thing, the Doctor and Rose strands without the TARDIS — on a planetoid that really has no business hanging so close to a black hole. A small world teetering on the edge of certain and utter destruction, while the population assure themselves "we don't know how this works, but it'll be fine". And they rely on a race of "willing" slaves to do menial support chores. Perfect metaphor for Western civilisation, really.
But add to that the occult-tinged threat of the presence, voiced by Sutekh himself, Gabriel Woolf, that possesses the Ood and Toby. There are decidedly Lovecraftian themes at play here that sit really uncomfortably in a Doctor Who story, certainly in 2006. Of course the inferior, "mindless" slaves are affected, like racist old HPL would like to imagine, but the real trespass is the corruption of a white researcher who studies eldritch knowledge and is taken over by primal forces (probably sexuality).
Up to now, the modern show has thrown ghosts, werewolves, and walking dead at us, with a comforting "—but it's aliens" twist to moor it in (light) science fiction. Here, like with the physical situations of the castaway Doctor and the impossible planet, there is no safety net to keep the storyline from plunging into superstitious darkness. It's like Event horizon, barely adapted for a family audience.
So this is a solid first-of-two episode with high stakes on several levels; as high as one can expect in a show where you sort of know they'll be okay at the end. The guest cast do a great job of presenting a convincing, everyday work situation. Everybody is weary and a bit jaded — even Ida opening the dome to reveal the black hole comes with a tired sigh as if to say, "oh sure, I'll show off that old thing again" — but you get the impression they've worked together for a while.
As first halves go, this has to be top shelf for modern Who. The followup — let's talk about that next week, don't want to ruin the good mood in advance.
Well, same voice actor. He's done quite a bit of extended universe things, but on screen (if that's the right term) he's been on the show 1975, 2006, and 2025. That's some staying power...