Anyone hoping for the tense, imaginative style of Se7en or Fight Club will be put out by David Fincher's long exposition of Robert Graysmith's book about the serial killer who terrified San Francisco in the Sixties and Seventies.

Deserting his usual pyrotechnical methods of heightening drama and creating tension, Fincher presents us with a careful, thorough investigation into the still unsolved case of the murderer who taunted the police with ciphers and letters for 20 years after the killings.

The results can be engrossing, but also more than a bit sluggish at times. It's almost as if Fincher wishes to prove he can tell a story dead straight. And straight it is, if never inept or badly made.

Graysmith (Jake Gyllenhaal), a newspaper cartoonist and serial puzzle solver, obsessively hunts to uncover the Zodiac killer's identity, interviewing the few victims who somehow escaped his clutches, poring over old files, following abortive clues and annoying both his employers and the police by nudging them towards his own theories.

Then, around a third of the way through the film, the killings stop. While the murderer is active, we're in the realms of a tense, well-made thriller. After that, it's a bit of a trudge towards Graysmith's favoured conclusion: that Arthur Leigh Allen, who died of a heart attack before the police could gather enough evidence to charge him, was the guilty man. Since that evidence was circumstantial, the police might never have charged him anyway.

Whether all this is worth more than two and a half hours depends upon your view of thrillers in general and police procedurals in particular. But be assured that Fincher orchestrates the period well - which progresses from 1969 into the Nineties - and uses his good cast to his advantage.

Gyllenhaal is at his best when Graysmith's efforts to solve the crimes and write his book reach what looks like some sort of conclusion. Mark Ruffalo and Anthony Andrews are solid as the leading detectives who doubt him. Robert Downey Jr plays his friend, an eccentric and sozzled crime reporter, with his usual, slightly camp, veracity.

In all, I have to say I preferred Spike Lee's Summer of Sam, another long movie about a real-life serial killer. But if you have more patience than me, Zodiac (which is also competing for the Palme d'Or in Cannes over the next two weeks) will reward you with its meticulous version of the cloudy truth about America's Jack the Ripper.