In one consciously ironic scene in Zodiac, a number of characters, including the lead detective investigating the notorious San Francisco serial killer, attend the 1971 premiere of Dirty Harry, loosely based on the actual case. The movie within the movie underscores the differences between the two works and, more important, the differences between fiction and reality, cop flicks and genuine police work. In Dirty Harry, Clint Eastwood nails the serial killer in properly dramatic fashion; in real life, the case remains open, full of unresolved questions and countless ambiguities.

At the theater, Robert Graysmith (Jake Gyllenhaal) introduces himself to Inspector David Toschi (Mark Ruffalo), beginning an odd, completely unofficial collaboration between the detective and the political cartoonist for the San Francisco Chronicle. The meeting between the two principals, which actually happened as the movie shows it, occurs well into Zodiac, a couple of years after the murderer embarked on his killings, taunting the media and the police with threats and bizarre letters and ciphers, and claiming credit for a dozen or more deaths. As it turned out in reality and in the picture, both the detective and the journalist fall into the grip of an obsession with the crimes, a circumstance that changes their careers and their lives.

Unfolding in chronological order with the calendar carefully noted on the screen, the movie covers in considerable detail the killer's actions and the progress of the investigation. Letters to the newspapers from someone calling himself Zodiac, accompanied by mysterious ciphers, boast of responsibility for the deaths of two couples, revealing details that only he could know. The press and the police, the two real subjects of the film, begin a long, exhausting, and ultimately frustrating search for the writer, who mocks their efforts for years and continues his murders.

The police procedure in Zodiac works rather differently from the brilliant scientific investigations of popular films and television shows. Attempting to find any clues to the killer's identity, both the cops and the newspapers consult handwriting experts and three government agencies specializing in codebreaking. The graphologists can reach neither agreement nor conclusions, and a retired school teacher solves the cipher that stumps the professionals, one of a number of examples of the official ineptitude that marks almost every investigation of a serial killer.

Another amateur, the cartoonist, also figures out the code and its sources and, working by himself, begins his own examination of the evidence, a process that as the years go by takes over his life. Like a number of others involved in the case, he becomes obsessed with Zodiac, who wrecks the careers and ruins the lives of almost all the people involved in the case. The Chronicle's ace crime reporter, Paul Avery (Robert Downey, Jr.), ends up destroying himself with cigarettes, booze, and drugs; Inspector Toschi's partner (Anthony Edwards) resigns from the homicide division and Toschi himself, once a celebrity cop, becomes the subject of a police inquiry; as Graysmith's obsession increases, his marriage disintegrates.

The slow, careful pace of the investigation itself depends upon the accumulation of minutiae from a variety of sources. Small, contradictory clues emerge; witnesses tell differing stories; the police make a number of dumb mistakes; disputes over jurisdiction impede almost every operation; and just about every cop, sometimes for good reasons, has his own favorite suspect. It seems not at all surprising that the Zodiac case remains open and officially unsolved.

The film's documentary approach suits the subject and probably demonstrates a far more accurate picture of police work than most examples of the cinematic procedural. With the exception of Robert Downey's jumpy, febrile performance as the eccentric wastrel Avery, all the actors, appropriately, underplay to the point of lifelessness, while the script maintains a resolute dullness of dialogue and characterization. Although the sheer development of the narrative and the accumulation of facts achieve their own energy, the essential inefficiency of the script prevents a compelling story from achieving the fullness of its promise, suggesting once again that life is bad art, or at least not the art we'd all like it to be.

Zodiac (R), directed by David Fincher, is now playing at Culver Ridge 16, Pittsford Cinemas, Henrietta 18, Webster 12, Tinseltown, Greece Ridge 12, and Eastview 13.