This is the only episode of this series that I have watched, and it didn't make me want to see more of the series or the recurring characters. It did make me want to see more of the non-recurring ones, though !
I found the main characters unremarkable, and their scenes not exactly boring but not especially interesting either : the good guys being the good guys, nothing we haven't already seen dozens of times.
By contrast, the villains are striking and memorable. They may not be the most complex characters in TV history (and how could they, in only one episode whose screentime must be shared with the main cast ?) but they don't need to be. I was actually surprised at the amount of screentime they get, which is much more than usual for one-time villains. This episode is centered around them, not around the main characters. Which is great, since they are wonderful to watch, much more than the main cast. I don't think this is the actors fault : the main characters are standard TV good guys doing standard TV good guys things, which I have seen so often than I was already sick of it by the time I was 12 years old. The villains have much more freedom, and when they are written by good writers and played by good actors they are especially great to watch... and here, they have one of the greatest. I will admit that I am totally biased when it comes to Jeremy Brett, and I can like even an awful film if he is in it (except Mad Dogs >_<). But his presence is not the only thing making this episode interesting, because the other two villains of the episode are also great. Their interactions together are what make this episode great, at least for me. The interactions always feel genuine : those between Jeff (Jeremy Brett) and Nancy (Hilary Tindall) are fascinating, like watching a train about to crash, and those between Jeff and the third villain (no, I won't spoil it ^^) were a big surprise the first time I watched it. Not the reveal itself, though it too was surprising, but the way the two characters interacted with each other : usually, if you see a manipulative character in TV or film, he will be manipulative with everyone (and a nice character will be nice with everyone, a sarcastic one will be sarcastic with everyone, etc) because of unimaginative writing and a tendency to oversimplify things. Therefore it is nice, for a change, to see the same character be remorselessly manipulative towards one lover whom he wants to use as a scapegoat, and yet also be totally genuine with his other lover. It makes for more depth, and Jeremy Brett is great at portraying it.