Wrapping up the first season of the Fourth Doctor Adventures, this also serves as the follow-up to the previous audiodrama, "Trail of the White Worm". It starts off as a bit similar to the plot of "The Avengers", of all things, before taking a bit of an unexpected turn. Sadly, the turn that it takes is a bit of a reminder of the limitations of audiodrama.
I'll refrain from too many spoilers once again, since it's more fun to see how the pieces fit together in terms of the overall continuity, but a large part of the story involves android copies of key characters. It also involves a series of feints and turnabouts, especially in the second half. The net effect is that there is a stretch of the story where it becomes difficult to keep track of who is real and who is a copy. And since the story hinges on keeping the truth close to vest, there are moments where the confusion is intentional.
It also serves as a bit of a sequel to "The Android Invasion". While that serial contains one of the more iconic moments of the Fourth Doctor era, the plot is an absolute mess. So the return of the Kraals is not exactly the most welcome turn of events. The story also involves UNIT, which makes sense given the callback to the previous story, but seems a bit dissonant, since UNIT is more closely associated (at least in my mind) with the Third Doctor. And the narrative gymnastics required to explain the absence of familiar UNIT characters is more sad than amusing.
Because the Fourth Doctor Adventures are set to return to this time period in its third season, arriving on digital shelves in 2014, there's little in the way of closure of any character arcs. Though Leela's "education" is more prominent a focus in these adventures than most of the actual Classic Who serials, it's clear that the Doctor's mission to civilize is ongoing by "Horror of Fang Rock". And thus, it's going to remain something of a pretext for how the Doctor and Leela enter into a situation, rather than a progressive story element. Which, while entertaining enough, does feel like a missed opportunity.