So: that one TV special that you didn't know existed? You can skip it, it isn't very good.
I was pretty psyched when I saw that there had been an adaptation of "Dirk Gently's Holistic Detective Agency." I haven't read this in, oh, probably about fifteen years, but still have many fond memories of it. It's a stand-alone title from Douglas Adams, and it brings the high level of wit from his early "Hitchhikers" books to a quantum-noir setting.
Adams wasn't just a great writer, he was also a really bright guy, and interested in all sorts of science and technology stuff. (Fun bit of trivia: he was the first person in England to buy a Macintosh computer. Stephen Fry was the second.) HHG2G was relatively light on the "science" part of "science fiction"; weirdly enough, there's more science in DGHDA, and arguably also in his proto-Gaiman mythmaking epic, "The Long Dark Tea-time of the Soul."
Sadly, it almost all got washed out in the adaptation. BBC4 put together a one-hour special that retains just enough from the original novel to make you miss it. Nothing is particularly bad about it, but it just doesn't seem very worthwhile, either.
The very, very, very broad strokes are still intact. Dirk Gently is still a detective who uses holistic methods, based on his theories about the fundamental interconnectedness of all things, to solve cases. Those cases tend to involve missing cats. He submits outrageous expense reports. And there's a fairly flexible timeline.
Other than that, it isn't very recognizable. Like I said, I haven't read the book in a long time, but I still vividly remember some of the best parts of the book, and am disappointed that they didn't make it into here. They really tease you, too: at one point Dirk orders a new fridge, and we can see the deliverymen carrying it up the stairs. Sadly, the payoff never materializes: the fridge just ends up in his office.
I almost wish that the show had been actively bad. The original BBC adaptation of HHG2G was laughably awful, and that's the key: it was so low-tech and scrappy that I kind of loved it. It's harder to do a fun low-budget detective story than a low-budget sci-fi story, though. Instead we get something just kind of "blah."
The biggest bummer, of course, is that since this wasn't all that good, it'll probably be even longer until we see quality productions based on Adams' excellent source material. It's kind of strange, since Adams had so eagerly engaged so many mediums so early in his career. HHG2G alone has famously existed as a radio program, series of novels, TV show, computer game (a text adventure, no less!), and Hollywood movie. Why can't we get a great stage play of Dirk Gently? Or a Terry Gilliam-directed film of Tea-time?
Ah, well. At least we'll always have the great stories he left behind.