Doctor Who: Phantoms of the Deep

In he year 2040, a scientific exploration submarine descends into the Mariana Trench, and, after picking up the Doctor and Romana en route, discovers an impossible undersea environment that soon proves deadly.

This is by no means the first Doctor Who story to be set in the depths of the sea, rather than outer space, but it does have the advantage of being able to use the environment more effectively than a TV budget would allow. TV story Warriors of the Deep, for example, is largely set on an underwater base, and much of it would probably work as well if the base were on, say, the moon (with an appropriate change to the monsters, obviously). Here, the fact that we're on Earth, in an environment full of life yet barely ever visited by humans, is a key element of the plot, and there are plenty of scenes involving swimming or deep-sea creatures that would be difficult to carry off on TV.

Unusually, the alien threat at the core of the story is never named - one character refers to them as “goblins”, but the others seem to reject that name - and, is for the most part, a shadowy presence. It's also unusual that K9 gets to do more than Romana, having a key role in the story, while she is somewhat sidelined. Having said which, the half-way cliffhanger in which she faces apparently certain death is a particularly strong one.

There are a few plot holes here and there, but the exotic setting and the odd nature of the aliens more than make up for that. There are also good performances from the supporting cast, notably Alice Krige (who played the Borg Queen in Star Trek, among other roles) as the expedition leader. Like many of the Fourth Doctor audio adventures, it feels a little short, but it is, nonetheless, one of the better ones.