Every Friday night lately, I've been glued to the television, for Ugly Betty is back on the air, and I just can't get enough of that brave little toaster and all her marvellous melodramas. I am well and truly cliffhung, and when I heard the second series was airing, I cheered.

This is the kind of viewer response that gets the ratings that lead to second series being commissioned in the first place.

But here's something I seldom hear from devotees of a television series: I want Ugly Betty to end.

Don't get me wrong. I love that show. But I don't want it to carry on indefinitely. There are two basic reasons for this:

1. I want it to end before something comes along and cocks it up.
2. I want an ending to put me out of my misery.

To deal with these questions one at a time. The first point is fairly simple, but is something that, because of the economic pressures of producing television, series seldom tackle well.

There's only so much life in any given story. Once character arcs have been played out, there's nothing more to do with them; once story arcs are ended, the story's finished. And yet the nature of getting series commissioned is such that the end of the story is not necessarily the end of the series. If it's still got good ratings, it gets stretched out; if its ratings are falling, it gets knocked on the head halfway through the plot.

This is nothing new. Old style penny dreadfuls ran under exactly the same system; with serials, it's a law of the marketplace. But alas, it doesn't make for very good art. Obviously it's frustrating to have your favourite series cancelled unfinished, but it's not that great to have it extended ad infinitum either. Nobody likes seeing sharks getting jumped, but if a story has to carry on come what may, after a while even the most resourceful writer may start casting longing glances towards the water skis.

There are series that can sustain themselves forever; CSI is a good example, having carried on for I don't know how many episodes and spawned at least two spin-off series that are very similar, not to mention many imitators, all without any sign of losing viewers or respect. The reason for this, though, is that it's highly formulaic. This in itself is an accomplishment; while I seldom watch it, I take my hat off to the producers for creating so durable a formula. You have established characters whose personal lives are unimportant enough that they won't confuse any new viewers, in a scenario that's easily grasped and, more importantly, generates infinite new plots. Just think of a new way to kill someone, or a new subculture for a death to take place in, and you've got a new episode. It's highly unlikely that CSI is going to produce an episode that's the most groundbreaking, innovative, startling half hour the televisual world has ever seen, but it is capable of carrying on for years and years without showing much sign of flagging.

But what if you have a series that includes an over-arching plot rather than a sturdy formula? At this point, you're picking a fight with Aristotle: there needs to be a beginning, a middle and an end. If you have a beginning, and a middle, and then some positive ratings figures, and some more middle, and some good ratings, and some more middle ... well, it's unsatisfying. The series can't go on for ever. The story questions it began with will have to be answered somehow, someday, and in the end, it'll become repetitive or tacky. And what happens? Audiences lose interest, and stop watching. It can turn into a vicious circle: if audiences start to fall away, writers may come under pressure to keep catering to the hard core that are still hanging in there - either pressure from studios, or just out of gratitude to the remaining viewers for keeping them employed - but the hard core are likely to have gotten attached to the characters to the point where they have difficulty seeing the wood for the trees. The result is often a show that becomes closer and closer to soap opera, the characters' relationships with one another becoming more and more important at the expense of whatever situation the story was supposed to be resolving, none of which can be expected to engage new viewers who aren't already interested in the characters and won't get interested when all that's left is soap. This inevitably fails to revive ratings, because the show hasn't regained its original charm, so they keep dropping, and the series gets the axe - conceivably in mid-story.

And when that happens, what you're left with is a tale that ends with a whimper rather than a bang. If you look at the complete piece, it won't satisfy you; it'll be a disappointing work of art.

It's for this reason that I prefer to think that series I like won't go on forever. I'd rather they were good in my memory as well as at the time, and remembering something short but great is more satisfying than watching a dying lion.

The other reason I'd like Ugly Betty to finish well is simple suspense. The story is based on cliffhangers, crisis after crisis, brilliantly sustained - but I like all the characters enough to hope it'll work out well. If it goes downhill to the point of unrecognisability, that won't be working out well: the issues that engaged my attention will never have been resolved skilfully enough to satisfy the tension that they created. I'll be left permanently wondering what would have happened if the series had ended well, which is as bad as wondering what would have happened if it hadn't been cancelled.

There is a solution to this problem, but it places tremendous demands on the writer: you need to factor into your plotting the points at which a series could be axed, and have some kind of resolution ready for each of them, while not packing every possible resolution into any one point. Given its superior quality, it's not too surprising that The Wire is the only show I can think of that's managed to do this. The first series is self-contained; the second two back-end each other, and the fourth, which isn't out on DVD yet so I haven't seen it, apparently moves away from characters who've served their turn and starts completely afresh, which is far better than flogging a dead horse. This is how it should be done - but it's difficult, and few series seem to manage it.

I have some hopes for Ugly Betty; so far it's suitably ruthless in dispatching or manhandling characters who have finished their story arcs, and a show that cares more about the story than the characters is a rare gem - and in any case, it was originally written as a soap opera, so may survive the soapification problem better than usual. But still if any network execs are reading this: please, please don't stretch it out until it asphixiates. Just give it a good clean death, and I'll watch the next thing you commission, I promise.