Adultery, murder, mayhem - and you call that cosy?

Her stories are dark enough already, says Laura Thompson. So why update Agatha Christie for TV?

'Cosy" is a word that I hate, and I am always irritated when I see it applied to the works of Agatha Christie. Actually I have always found her books rather bracing in their worldly attitudes towards human nature. Nothing especially cosy about adultery and cupidity accepted as a fact of life, about children who kill and are killed, about a man who murders in order that his ex-wife should hang for the crime. Yet that is the stuff of Christie's detective novels, contained though it may be within her highly structured sense of the proprieties.

Her 1945 book, Sparkling Cyanide - newly dramatised, and broadcast on ITV1 tonight - has greed and lust as its subject matter. I can remember being slightly shocked when I first read the book because of the way in which Christie described a married woman flirting and dining and sleeping with another man: this seemed to me extraordinary - prim little teenage bourgeoise that I was - but cosy middle-aged Agatha wrote about it as a matter of course.

Naturally the passions that lurk within the novel are held at a ladylike distance. Rumpled beds loom only in the imagination; the only social solecism committed is that of placing lethal poison in the champagne glass of one's dinner companion. Yet there is a wealth of knowledge about human nature. And there is nothing cosy about that knowledge, for all that it is deployed with a little finger lifted away from the sordid realities.

So I was rather pleased to learn that Sparkling Cyanide had been transposed to the present day. This is the first time it has been tried with a British television production of a Christie story. No doubt producers here have been terrified of alienating what is seen as her typical audience. Which is why we have had that interminable series of Poirots, with its twee music, Art Deco sets, and sub-Ronald Colman/ Gertrude Lawrence acting.

Poirot plods gamely on (has David Suchet not solved everything yet?); clearly it is pretty popular among the cosy brigade. But there seems to be a feeling that it is time to move some of these adaptations into a different milieu, to test them out in what we would call "the real world". The books can surely take it: like their creator, they are robust creatures. And so this new Sparkling Cyanide has the blessing of Christie's grandson, Matthew Prichard, who is also chairman of Agatha Christie Ltd. "I am convinced (it) will do full justice to my grandmother's work and underline her relevance for a 21st century audience," he has said.

Agatha Christie is, indeed, still relevant: her books continue to be devoured with fascination and pleasure. "They sell very well, and not just to tourists coming out of The Mousetrap," says Maxim Jakubowski, who owns the Murder One bookshop on London's Charing Cross Road.

What seems less relevant, quite frankly, is the all singing, all dancing, all spin-doctoring setting that Sparkling Cyanide has been given. It really doesn't need its central characters to be turned into what pass for modern archetypes: the wide-boy football club owner, the personal trainer, the cabinet minister with the QC wife and the dirty secret. The dramatis personae of the book could have been brought into 2003 without that kind of overt tinkering. Lust and greed, love and hatred, are eternals after all; Agatha Christie knew this perfectly well; the adaptation should have trusted her more.

For she is, to my mind at least, a vastly under-rated writer. Of course the fashionable view is that she is a supreme constructor of plots (and thus ripe for updating - all the hard work already done!), while a novelist like Ruth Rendell is far superior when it comes to understanding, and conveying, the motivations that impel the plots. Rendell herself appears to agree with this judgement since she has dismissed the Christie oeuvre as formulaic and her characters as ciphers. This is superficially true and fundamentally wrong. If Christie's books were no more than puzzles then they could never have endured the way that they have.

In fact, although she was not an artist in her writing she was most certainly one in her thinking. Her mind worked in such a simply clever way. We have grown so used to her books that we no longer see them for what they are: the products of a remarkable creative sensibility. And the formal shapes, the near-mythical resonances, the iconoclastic twists of Christie's detective fiction are wholly artistic in conception, however this is concealed beneath the plainest of prose.

Although in her less good novels there is such a thing as "mere" plot (this is true, in fact, of Sparkling Cyanide), in her best work the plots are a geometric expression of something much deeper. Take, for example, Murder on the Orient Express, whose plot resolution implicitly poses the question "what is justice?" Take And Then There Were None, whose sweepingly bold central premise examines the whole notion of guilt, and whether it is possible to live with it. Take Crooked House, which is concerned with the idea of family, the good and the bad of it.

I don't mean to overstate the case. These themes are not treated by Christie as they would have been by Dostoevsky. She herself "loved simply to entertain", as Jakubowski puts it, and was happy to describe herself as a craftswoman ("one up for the lowbrows!" she is said to have cried, when she received the CBE), saving her more expressive writing for the six, very good novels that she wrote under the pseudonym Mary Westmacott. Nonetheless the quality of her thinking is there in her detective fiction. It underpins and solidifies her books, giving them a stern opacity that endures beneath her transparent style.

What is essential to her work, of course, is the element that has become traduced by the dread word "cosiness". If I say that the dark knowledge of human nature within Agatha Christie is restrained by her sense of the proprieties, that is not a criticism: far from it. It is profoundly necessary to her appeal.

And herein lies the danger of updating her: when the workings of her plots are let loose within a less restricted setting, they lose a great deal of their power. Tempting though it is today to shy away from her "snobbishness", it may well be that if you take out the parlourmaids you take the heart out of Christie.

For she understood (as did George Orwell) that murder - surely the ultimate example of the human will to shape destiny, certainly the most devastating act of which we are capable - is at its most interesting when carried out within a framework of restraint, of conventionality. Think of Crippen, waving cheerily to his neighbours, promising to send their regards to the wife that he had buried under the cellar. What more perfect representation of the Christian sensibility, the one that venerates the potency of the ordinary, of the status quo that murder destroys?

Hence, I suppose, cosiness; or what Maxim Jakubowski calls "bourgeois-lit". It is "cosy", in a sense, when the old lady detective Miss Marple says - over a cup of tea - that it is always best to think the worst of human nature, since the worst so often turns out to be the truth. At the same time it is not cosy at all. It is frighteningly realistic, shockingly so in an age like ours, intent as it is upon positivity: the benign, unyielding morality that runs through Agatha Christie's books is not at all the kind of thing we go in for today. For her own part, I would venture, she would scarcely have approved the sympathetic treatment that tonight's film of Sparkling Cyanide gives to the murderous culprit.

Which is why updating is problematical. We are wary of her steely structures; we are equally nervous of her worldly wisdom. And yet: imagine what could be done with a book like And Then There Were None. Imagine it adapted seriously, stylishly. Not with the characters turned into PR girls and coke-snorters but into - dare one try this? - real, complex human beings. The books, as I say, can take it. I should also say that they deserve it.

'Sparkling Cyanide', ITV 1, 9pm, today. Laura Thompson is author of 'Life in a Cold Climate: Nancy Mitford - A Portrait of A Contradicitory Woman' (Review £20)