Endeavour (ITV1)

When you've run out of ideas, don’t write a prequel, suggests Rachel Cooke.

Oh dear. I hear the sound of a barrel being scraped. First we had Inspector Morse, which was really rather good, and so much better in every way than Midsomer Murders (or Moga­don Murders, as the writer Roger Lewis has it). Then we had Lewis, in which Kevin Whately did his plodding, exasperated thing all over again, only this time with Billie Piper's husband rather than the über-crotchety John Thaw, who died in 2002.

No doubt the producers are already hard at work on a series called Hathaway (for this is the character played by Piper's husband). But in the meantime the writer Russell Lewis has come up with Endeavour (2 January, 9pm), a Morse prequel. Me? I'm unhappy about it.

As a Morse fan, I don't want certain things spelt out. I prefer to imagine the backstory - the pain, the hurt, how our hero's slow calcification began. Morse's touching Quaker first name was a secret until Death Is Now My Neighbour, the twelfth of the Colin Dexter novels in which he appeared, and yet this Russell Lewis person has had the temerity to use it as the title of his film (which he no doubt hopes will lead to a commission for a series). It's so very wrong, like catching sight of the Queen in a cutaway swimsuit.

But I must try to calm down and do my job. It is 1965, and our hero - a wan, dreamy fellow played, with a slight Scouse accent, by Shaun Evans - is embarking on a police career in a place called Carshall Newtown where gargoyles are exceedingly thin on the ground. He doesn't like it much, but before he is able to resign, he is called away to Cowley, Oxford, where a girl has gone missing and more manpower is needed.

How does Morse feel about this? A little queasy. He left the place only shortly before, having failed to sit his finals. As a friend from his undergraduate days at Lonsdale College will tell him, when they inevitably collide in the front quad: "Poor old Morse, you never were Oxford material - too bloody decent by half." Young Morse doesn't snort at this as Old Morse would have done (not to mention any sentient viewer at home). He merely looks all pale, anxious and intimidated. What a milksop. When he went to the Lamb and Flag and had a pint (glass with a handle, natch), I was amazed. I was half expecting him to ask for a port and lemon.

What followed was creaky and predictable: snobbish dons, a vengeful wife, crossword clue secret messages, opera on the turntable, plus a bit of corruption back at police HQ - this is the Sixties, after all, when spivvy men with pencil moustaches, just like cabinet ministers, will take their pleasures where they find them. Not Morse, though, who is so angelic that he weeps over the body of a murderer.

Luckily, his patron, DI Fred Thursday (Roger Allam), is not only as honest as the day is long; he is kindly, too, even when the new boy starts wittering on about the poetry books he found on the victim's bedside table. Man, all this irritated me. I know when I'm being palmed off with second-rate goods - and I bet most other Morse fans do, too.

Yet much worse were the film's attempts at sophistication. When Morse was shown to his dreary new lodgings in Oxford, the landlady said: "This was Mr Bleaney's room," - a steal (from Larkin) so obvious, surely no one alive would feel flattered to have recognised it. The producers also went half-heartedly after a certain knowing ghostliness. Abigail Thaw, John Thaw's daughter, who had a tiny cameo, asked Morse if they'd met before. "In another life," she smiled, when he said not.

At the end of the film, Morse looked in his rear-view mirror and John Thaw's eyes stared momentarily back at him. How utterly weedy. Why, I wonder, was this film given the green light? The word "prequel" - like the phrases "meat flavour" and "leather style" - should sound its own warning. But obviously there's no accounting for the arrogance of a certain kind of television person. Think of the best thing you ever saw on TV - Brideshead Revisited? The Jewel in the Crown? - and know that, even as I write, someone somewhere will be busy trying to improve on it.