DAVID FINCHER'S new film Zodiac tries hard not to be a serial killer movie. So on that count, it gets off to a bad start. The opening scene shows a couple, parked in a lovers' lane, being blasted by a gunman who emerges from the darkness of a prowling car. As sequences go, it is so stylish and so well executed, you are slain in your seat before the first mouthful of popcorn. The Zodiac, a serial killer who terrorised San Francisco in the late '60s and 1970s, but who was never caught, strikes twice again. The scenes take the rhythm of the attack: the drawn-out horror of being punctured repeatedly by a knife, or the efficient closure of a bullet to the back of the head. When David (Se7en, Fight Club) Fincher goes in for the kill, there is no director more capable of instilling that can't-look, must-look awe in the viewer.

But these scenes also create a tension in the film that is the result either of Fincher's need to show off, or play a perverse trick on the viewer. For while you witness the murders, you never identify the killer. And this is a tease. (There is a goose-chase genre brewing in Hollywood:

period films about unsolved cases such as Hollywoodland and The Cat's Meow produced similar frustration. ) Zodiac instead wants to be about the obsessions of three men, played here by Jake Gyllenhaal, Mark Ruffalo and Robert Downey Jr, who tried unsuccessfully to find the killer, and ruined their personal lives in the process. So it's a procedural without a resolution. And the viewer's anxiety for closure feeds into the frustrated characters in the film.

Still, there is so much to savour. There's the performance of Mark Ruffalo, who eases into the middle of the film like butter in a pan. He gives it a soft sizzle. His detective looks like a smoothed-out Peter Falk in the same trench coat. And then there's the look and feel: a period film that captures a San Francisco in 1969 that is less summer of love than summer of fear.

This is the first Hollywood film to be shot entirely on a digital Thomson Viper Filmstream camera, and it looks sublime:

cinematographer Harris Savides has found a texture that looks rich and darkcreamy like '70s wallpaper. And the men who are unrolled as shiny-new at the start look ragged and discoloured by the end.

There's crime reporter Robert Avery (Robert Downey Jr), a druggy alcoholic with dandy flair (one suspects Downey Jr had little to do to research the role). He is put on the case by the San Francisco Chronicle when the Zodiac begins sending encrypted letters to the paper. But the affair will burn him out and he will finish his life in obscurity.

There's newspaper cartoonist and amateur cryptologist Robert Graysmith who lends Avery a hand. He becomes the thread of the film but is played with the least conviction here by Jake Gyllenhaal.

Graysmith's life-long obsession (his two books on the case provided much of the meat for the film) puts his family in jeopardy and breaks up his marriage to Melanie (Chloe Sevigny).

And then there's police inspector David Toschi (Mark Ruffalo), whose legacy will not be the Zodiac case, but that he was the inspiration for Steve McQueen in Bullitt.

Alas, there are no car chases here, and the leads soon peter out. His partner Bill Armstrong (Anthony Edwards) sensing failure, asks to be transferred. And then Toschi is wrongly accused of faking Zodiac letters to gain publicity. So the Zodiac wins. Sort of.

The film senses our need for resolution, and works something up in the shape of a staring match: Graysmith in the last stretch tracks the man he believes to be the killer down to a hardware store. Was it Arthur Leigh Allen (John Carroll Lynch), circumstantially the chief suspect who got away because fingerprints, DNA and handwriting evidence proved that he was innocent?

Where Zodiac excels is in its depiction of obsession to the brink of disaster, and in its recreation of human fog: the case was bungled by jurisdiction problems and procedure hampered in an era of paperwork mountains and a difficulty in getting a fax on time. And then there was the fact nobody knew just how many murders Zodiac even committed.

Fincher, a notorious perfectionist, has crafted an imperfect procedural, perhaps freighted with too much information, but the very opposite of Dirty Harry . . . the naughty spin-off based on the killings and referenced in a cardboard cutout here. It's an ambitious, riveting work and it should be read as a statement of intent: that Fincher is taking his kinetic cinema into a more mature place.