If my wife and I watch television, which is not often, then we tend to go for only the most challenging and intellectually demanding programmes. She has a weakness for Newsnight. I prefer The Simpsons. Between them, these two mighty programmes soak up a lot of our appetite for TV. But when you are as rigorous in your tastes as we are, there does come a time when you turn to each other and say: "Why don't we relax with something light-hearted tonight? Like murder?" So what we do, of course, is watch a bit of Inspector Morse. And if that is too challenging, we go down-market, murder-wise, and dig out a video of David Jason as Inspector Frost or George Baker as Inspector Wexford.

But sometimes even Frost and Wexford seem too much of a challenge. Sometimes we want something so artless, so simple, so watchable, in the sense that a goldfish bowl is watchable, that we occasionally turn to Midsomer Murders, as we did last Sunday, and watch Inspector Barnaby tackle murders that I occasionally think could only have been dreamt up by the British Tourist Authority.

If you have never seen Midsomer Murders, all you need to know is that these dramatised tourist tales are set in Touristshire, that mythical county of England where everything is thatched, mullioned or inglenooked. The houses are idyllic. The scenery is lyrical. But alas, the people who live in this timeless zone are not always worthy of it, for although they are paid-up members of the middle class, they also commit adultery, they have guilty secrets, and they kill each other. And then someone has to send for Inspector Barnaby and his sidekick Sergeant Troy.

But Barnaby has a problem of his own. His problem is that he is played by John Nettles. And Nettles has decided to act this part as if he knew that my wife and I do not want to be made to think too much, and he will do all the thinking for us. So he tends to allow only two main expressions to settle on his face. One is a sort of puzzled determination. So is the other.

(The Radio Times agrees. Previewing the last of the current series, Alison Graham writes that when Barnaby's wife is trapped in a canal tunnel, he "seems to be taking it personally, though it's hard to tell from his expression, which hovers somewhere between mild annoyance and mild curiosity.")

Of course, it may be that the rueful Nettles expression is a permanent condition brought on by the plot lines. An order has presumably gone out from the British Tourist Authority that each episode must feature several British heritage attractions, and thus last Sunday's offered us:

a) A canal restoration project (industrial heritage)

b) Bodies buried in the canal workings (family history)

c) Murder victims found in lush parkland (dangers of trespassing)...

d) ...With their feet caught in vicious and deadly mantraps (the reliability of English workmanship)

e) And a tramp who is so in tune with nature that he is followed by tame owls, a deer and a faithful fox and its little family (rural conservation).

(This last caused my wife to choke with laughter. The sooner she gets back to Newsnight the better.)

But what was significantly different about this episode was that the coterie around Inspector Barnaby was beginning to show odd signs of independence. There was his wife, vanishing into the canal workings, trapped underground, absent for most of the episode. His daughter, nearly murdered by the killer. And Barnaby's sidekick, Troy, getting promotion and due to be transferred north, never to be seen in the series again. It's almost as if those nearest to Nettles are doing their best to distance themselves from his unnerving gaze.

Maybe, in the next series, Touristshire will be inhabited only by the dull-eyed form of Inspector Barnaby, trudging round the exquisite landscape, looking for company and finding only cadavers of people who did away with themselves when they saw him approaching. I can't wait to find out.