David Fincher does a stand-up job of recreating 1960s San Francisco in this meandering real-crime thriller, but at two and a half hours it is simply too drawn-out to be enjoyable.

Jake Gyllenhaal plays Robert Graysmith, a cartoonist on the San Francisco Chronicle who becomes increasingly obsessed with the activities of Zodiac, a serial killer with a penchant for confessional letters. He sees the letters at first hand when they arrive at the newspaper, hovers around the desk of the crime correspondent (Robert Downey Jr) and is ultimately moved to research the killings himself for what becomes his bestselling book on the unsolved murders.

But while the story unfolds fairly evenly and with an impressive attention to detail, it is hampered by the inconvenient truth that Zodiac was never arrested, let alone convicted. It rather limits the story's progression, given that the police are no nearer to catching their man at the end than they are in the middle. The result is an epic accumulation of circumstantial evidence that, despite heavy doses of foreboding bass notes, is mildly intriguing rather than exciting.

The last time Hollywood had a go at making a film of the Zodiac killings it came up with Dirty Harry, whose titular hero takes down his quarry with as much observance of due process as his .357 calibre Magnum will allow. Naturally, a realistic portrayal of the story isn't going to feature such pyrotechnics, but watching the cops capitulate to the goddam pencil-pushers at City Hall does not make for riveting viewing.

Not that realism prevents Fincher from nailing his colours firmly to the mast when it comes to his preferred suspect, mind. I was almost convinced myself until one of the closing captions mentions that a recent DNA test all but exonerated him. But still, he's dead, so he's hardly in a position to sue.

Gyllenhaal and Mark Ruffalo, as the dogged cop leading the investigation, put in creditable performances here, but Fincher should have known that the story was only worth 90 minutes.