No, the main mystery with this is why Robert Holmes, a man who worked on the production side of things and was around for stories like Invasion of the Dinosaurs and Robot, was unable/unwilling to convince Graham Williams and script editor Anthony Read that a story about a giant monster was a bad idea. But no, instead we get a story with gun-running rogues, green natives, and a truly gigantic squid (the realization of which in part two is a hell of a lot better on the picture-stabilized DVD than on the original).
To be perfectly honest, though, these first two episodes aren't that bad. All the swamps and high reeds do make this looks like an alien moon, and it's nice to have one that's not in a quarry. And Tom Baker seems a bit more reined in on these two episodes (slipping cups of liquid into his pocket aside), which does sell the threat a bit more. And the green makeup on the native "Swampies" is surprisingly effective. And somehow, they've managed to put together a top-notch cast, with Philip Madoc, Neil McCarthy, and Glyn Owen. They've even gotten John Abineri as Ranquin, the leader of the Swampies. This is a talented cast, to be sure. The problem, though, is that these two episodes are a bit dull. There's some stuff about sending guns to the Swampies that looks like it was meant to both give the Earth colonists a reason to kill them all and discredit a troublesome organization at the same time, but it's handled in such a way that it's difficult to care. The scenes in the methane refinery are slightly more interesting, but they're just tracking the strange movements on the sea bed that turn out to be Kroll, a gigantic squid who happens to be waking up after a 200 year slumber -- there's little else going on. Really, it's only the scenes with the Doctor that are really watchable, and he spends a good deal of time being captured.
Any good bits? The Kroll monster costume that turns out to deliberately be a costume is a nice touch, and the model of Kroll is actually rather good -- it's simply the mix between the effect and the film that doesn't work (and even if the cameraman hadn't physically masked off the top of the shot while shooting, thus making a very obvious straight line across the screen, it's unlikely this would have really worked). And it's nice to actually get to see John Leeson, the voice of K-9, for a change.
Like I said, the worst thing about these two episodes is just that they're a bit dull -- even the usually reliable Philip Madoc seems to have trouble delivering lines in an interested manner. But the cliffhanger to part two, where a rather unconvincing rubber tentacle breaks through a pipe and pulls one of the refinery workers inside, might be a worrying sign of things to come...