David Toschi, movie star of the San Francisco Police Department, is the fast-driving, quick-talking, quicker-thinking cat who inspired Steve McQueen's character in Bullitt, Michael Douglas' in The Streets of San Francisco, and Clint Eastwood's in Dirty Harry.

In Zodiac, Mark Ruffalo, rumpled, shaggy and suggesting Columbo more than screen star, plays Toschi, lead investigator of the murders claimed by the shadowy self-regarding figure who called himself the Zodiac. In the early '70s the serial killer held the Bay Area in his grip of fear.

Ruffalo has the same relation to prior screen versions of Toschi that Zodiac has to prior films by David Fincher, maker of Seven, Fight Club and Panic Room. Best-known for pulse-pounding thrillers with sufficient splatter for the bloodthirsty, Fincher has made Zodiac as procedurally meticulous as All the President's Men - and as enthralling.

It's not the sociopath who intrigues Fincher here. He's obsessed by the obsessives who want to solve the cipher that is the Zodiac as though he were a homicidal anagram or a killer Sudoku. As Fincher mordantly suggests, the fanatics who struggled to decipher the Zodiac are no less his victims than those the murderer actually stabbed and shot.

Such obsessives naturally gravitate to police investigation, investigative journalism and filmmaking - jobs in which it is the professional's task to piece narratives out of threads of story. Fincher spins these threads into a taut thriller, one without everything neatly tied together.