Director David Fincher's previous police procedural - Se7en - enjoyed the luxury of being a fictionalised account of a killer whose modus operandi worked around the seven deadly sins.

Here the story of his bloodlusting psycho is based on sober reality with no allowances for flights of fancy like Gwyneth Paltrow's decapitated head being Fed Exed to the investigating officer.

The restrictions work in Fincher's favour as he meticulously forges a never-less-than compelling yarn around Zodiac, a mass murderer who leaves enigmatic codes - a nasty sort of Sudoku - with the San Francisco press.

In American lore, Zodiac occupies the same sort of place as Jack The Ripper in London's East End, with a body count of five between December 1968 and October 1969.

Like all good serial killings, there didn't appear to be a motive for the murders, just a random set of slayings of victims unfortunate to be on there own in Californian beauty spots.

Zodiac's traditional weapon of choice is a 9mm Luger although he varies this in one terrifying scene where he trusses up a pair of young lovers on a lake shore and then bloodily perforates them with a hunting knife.

On his elusive trail is San Francisco cop Inspector Dave Toschi (Ruffalo), his partner William Armstrong (Edwards), crime reporter Paul Avery (Downey Jr) and his cartoonist colleague Robert Graysmith (Gyllenhaal).

Fincher's attention to detail led to him paintakingly sifting through 10,000 pages of police transcripts as well as interviewing all the surviving players. His obsessive research pays off with a finely wrought plot that shows Zodiac's victims weren't just the unfortunates he offed...but those in not-so-hot pursuit.

Toschi was taken off the case (and suspected of sending letters in Zodiac's name), Avery became a suspect and slipped into drink and drug dependence while Graysmith's marriage hit the rocks as his obsession took over.

If you're not a fan of police procedurals, the running time of almost three hours may wear heavy. But aficianados of the style will find plenty to mull over and enough material to keep conspiracy theorists foaming for a few years.

Tim Evans