WATCHMEN is a comic book and a work of art. The debate that statement entails is now more than 20 years old, and periodically dredged up by the conservative thinkers of Hollywood and professional geekdom ever since it became apparent that the "superhero movie" was a licence to print money. Yet in an age of wilful illiteracy, the idea of reading a book - even a comic book - when you could watch the film remains sacrilegious. Artistic concerns? How tragically quaint.

Although it's not the most compelling of Watchmen's virtues, this is a work that fulfils the promise of a medium respectfully known in France as "the ninth art". Unlike so many others - think of Mark Millar, a Scotsman whose comic book stories (Wanted, Kick-Ass) function just as well as Hollywood pitches - writer Alan Moore and artist Dave Gibbons fashioned their book as a showcase of techniques and devices that could be done nowhere else but in a "graphic novel". Not in a poem or a painting; not even a movie. And yet a movie version will be released in two weeks' time.

Originally, the plan was to use up some old and largely forgotten super-characters from the defunct Charlton Comics line, to which publisher DC Comics had acquired the copyright. The company offered the job to Northampton's Alan Moore, then regarded in the industry as strange but profitable after his work on 2000AD, V For Vendetta and Swamp Thing.

In a field littered with dystopias, Watchmen's narrative is set in a world not particularly better or worse than ours - but different. The fractured mirror-universe of 1980s America in which it takes place is summoned through stories, background details, old photographs, the detritus of consumer society and the shared neuroses of a generation. Ultimately, Watchmen is about what fear of the world's end can make of us.

Brevity forbids discussing Moore's narrative intricacies, Gibbons's flourishes of symmetry or even recounting the plot without leaving it hopelessly abridged; but it dazzles at every turn. The book creates a 1985 where Nixon is still US president, cars are electric, the cold war rages and Vietnam was won by a naked, blue, superhero god called Dr Manhattan. It presents a paranoid, right-wing sociopath who relishes violence and welcomes Armageddon, then makes him your favourite character. It evokes sympathy for a rapist. It tells a story about damaged people in masks and capes, watching the slow death of their dreams, and makes you care. Watchmen does damn near everything, but above all else, the comic surprises us.

It also encompasses murder-mystery noir, a meditation on the character of America, science fiction, several romances, a Swiss watch of a conspiracy, a host of weird and utterly convincing psychological portraits, and one of the great twist endings of all time. And throughout it all, Watchmen carries with it an intelligence, at once cosmic and caustic, that says more with allusion than other writers could say with entire novels, justifying every "fantastique" conceit the story unfurls.

Moore is a charming, powerfully intelligent gentleman who is offended by Hollywood, and who in turn offends studio bosses for having the gall to reject their advances when he believes his work should not be transferred onto the big screen.

Along with Art Spiegelman's Maus and Frank Miller's The Dark Knight Returns, Watchmen revolutionised the comic industry. One would have hoped the least Hollywood could have done was wait until Moore was dead before forcing his pictures to move. But few people in the film industry talk about the betrayal of the artist - and fewer people care.

Some years ago, professional lightning rod Terry Gilliam asked Moore how he would go about turning Watchmen into a movie. Moore replied, famously: "I wouldn't."

Good joke. Everybody laugh. Roll on snare drum. Curtains.

Watchmen is published by DC Comics/Titan Books, priced £24.99 (hardback), £17.99 (paperback)