During the first series of the Doctor Who revival, The Doctor and his companions never left earth. Somewhat understandable in terms external to the story, but a great loss in terms of what is possible with Doctor Who, which more than any show ever filmed is limited only by imagination. And costuming, prop, special effects and scenery budgets. In the second series, starring David Tennant as the 10th Doctor and with the continuing presence of Billie Piper as his companion Rose, there are three or four episodes that take place in alien locales. Along with "New Earth" and possibly "The Girl in the Fireplace", there is the two-part story "The Impossible Planet"/"The Satan Pit", which I will be treating in this write-up as one story.
The episode starts when the Tardis materializes on a broken down, battered exploration base on a small, uninhabited planet orbiting a black hole. With writing on the walls that is so ancient that the Tardis can't translate it. The exploratory crew of the base, which consists of a half-dozen people, assisted by a hive mind race called the Ood. The planet has a mythology built up around it, based on its unusual physics (orbiting stably inside of the event horizon of a black hole, using an anti-gravity generator) supposedly being an ancient trap built for some type of entity trapped within it. And over the course of the two episodes, we see that the legend is based in fact, as we see hints, and then revelations, that there is indeed a great and hidden evil. The slow revelation of the menacing presence trapped in the planet is truly creepy.
The episode certainly captures some great tropes of Doctor Who, including such perennial points as a race against the clock, a crumbling base, and lots of running/crawling down corridors. Also, of course, a gradual death toll, and a crisis that is only averted at the last moment. Several times.
In terms of the development of the series, the most important point of this episode is that it truly made the decision to go big. The episode takes place far out in space, in an undisclosed future time, and it deals with a menace from the deep past that, when we finally see it, is big in terms of physical size and presence. The entire episode is, in a word, cosmic. The layout of the base, and the appearance of the crew, would also become a design staple of revival who: futuristic technology that looks like it has been repaired with spit and baling wire. The entire thing is realistic and atmospheric and helps build the episodes tension.
If there is any trouble with the story, it has to do with one of the flaws of revival Who overall: the relationship between individual episodes and overall plot and character arcs. The Doctor and Rose visit their most cosmic location yet, face down an ancient evil... and then the episode ends and it is just another adventure, not much more different than meeting Charles Dickens in Cardiff. However, building story arcs is not an easy matter, and the fact that this spectacular story didn't quite fit into the rest of the series as well as it could have is a somewhat minor point.