But it's actually Turlough's efforts to destroy the TARDIS ("I am ready to lift you away," the Black Guardian tells Turlough -- the words "yeah, right" come to mind) that lead to the main storyline, as the TARDIS latches onto a nearby spaceship to save itself from breaking up. Nyssa heads through the doorway that appears, and the rest of the story so far is about what she and the Doctor discover. (Tegan and Turlough spend the end of part one and all of part two trapped in some ductwork beneath the floors of the ship. It's about as thrilling as it sounds.)
I do like the slow realization, near the end of part one, that this ship isn't simply carrying standard cargo, but instead plague victims, and so the two raiders, Kari and Olvir, learn not only that they've been abandoned on a ship by their captain, but that it's a ship full of diseased people. Or, as Olvir memorably puts it at the end of part one: "This is Terminus! Where all the Lazars come to die. We're on a leper ship! We're all going to diiiiiiie!!!" And we learn in part two that Nyssa has also contracted the disease and is thus forcibly separated from the others.
Of course, it turns out that there are other people aboard Terminus. The ones looking after the Lazars are the Vanir -- but rather than being like the Norse gods of their namesake, these people are a sickly lot, dependent on a drug called hydromel and looking little better than the patients they're handling. I do like the look of their armor though, with its skeletal motif -- but there are also some small little touches (a snake head here, a dragon there) that add some character to the look. And people tend to look down on the Garm, but, glowing red eyes aside, I've always found it to be a reasonably convincing character. The Garm is certainly imposing; it's huge in every direction, and the idea of its being a servant of the weak-looking Vanir suggests that they must have some hidden strengths. And we get to find that out in part two's cliffhanger, as one of the Vanir attempts to throttle the Doctor -- with hands around the throat this time, rather than the usual Shoulder Rub of Doom.
There are other nice touches scattered throughout these two episodes; the skull motif on the transport ship, the transparent colored blocks that represent computer chips (rather than the standard "bits of circuitry" we usually get around this time), and all the different colored strips that show that the Forbidden Zone has been slowly getting larger (presumably as the radiation contamination increases). It's been a slow build, but thus far Terminus has been an intriguing story.