This haunting film by David Fincher is both eerie and, at several points, really frightening, too — just what you want from a serial-killer movie. Its most remarkable accomplishment, however, is more low-key. Over the course of two hours and 35 minutes, the picture lays out an enormous amount of information about the Zodiac murders in the San Francisco Bay area in the late 1960s, a 10-month spree by a still-unknown assassin that became the basis for the 1971 Clint Eastwood movie, "Dirty Harry." "Zodiac" is packed with procedural detail, but it goes down smoothly — we never feel like we're being force-fed. And what the movie demonstrates most memorably is how an overabundance of raw data — police reports, expert speculation, conflicting witness statements — can spread not illumination, but increasing uncertainty.

Like Jack the Ripper, who despite his enduring renown actually murdered only five people during his brief rampage in 1888, the Zodiac, as he called himself, was a small-scale eliminator. He, too, is credited with just five killings (although there may have been more). But the reason his crimes continue to resonate is that, like the Ripper, he made the media an accessory to his depredations. The letters and mysterious ciphers he mailed to the San Francisco Chronicle and other local newspapers — usually beginning with the salutation, "This is the Zodiac speaking" — creeped people out in a serious way.

The movie begins on the night of July 4, 1969, in Vallejo, California, with small parties in progress and fireworks lighting up the suburban sky. At a deserted teen parking spot far from the holiday hubbub, a young man and woman (Lee Norris and Ciara Hughes) are sitting in their car talking. Donovan's "Hurdy Gurdy Man" is playing on the radio. Then, quietly, a second car glides up behind them. It just sits there. After a while, it drives off. An uneasy moment passes. But then, along with the two kids, we see headlights in the distance. The second car is coming back.

Fincher stages this first Zodiac attack with an admirable lack of pulp frenzy. We can't quite make the killer out, and he's no clearer in a pay-phone booth a bit later when he calls in his crime to the police. ("I also killed those kids last year," he tells the cops offhandedly.) The murderer subsequently mails letters to three newspapers, each missive containing part of a complex cipher which the Zodiac claims contains his identity, and which he demands that the papers print. At the Chronicle, this draws the interest of hard-drinking hotshot reporter Paul Avery (Robert Downey Jr., who's never been funnier than he is here) and a geeky, mild-mannered editorial cartoonist named Robert Graysmith (Jake Gyllenhaal). (The movie is based on two books about the Zodiac case written by the real Graysmith.)

The Zodiac's cipher turns out to be a stumper — the FBI, CIA and NSA all take a shot at cracking it, and all fail. Then an amateur code-breaker deciphers it. The murderer says he enjoys killing, and he believes that his victims will all become his slaves in the hereafter.

Fincher gets all of the picture's period details right — the jaunty little hipster neckerchiefs and thick leather watchbands; the big clackety electric typewriters in the Chronicle newsroom — without calling too much attention to them. And the pop music that floats through the film is dead-on without being groaningly clichéd. (Three Dog Night's "Easy to Be Hard" is just right for the opening scenes, and later on, so are the Oliver version of Rod McKuen's sappy "Jean" and the jailbait anthem "Young Girl," by Gary Puckett and the Union Gap.)

The story proceeds in a series of intense set pieces. There are the actual killings, of course — especially one on the sunny bank of a lake near Napa, which conveys an inhuman brutality without descending into witless gore — and also a frightening late-night encounter in a basement between Graysmith and a weird little man named Bob Vaughn (Charles Fleischer), who may — or may not — actually be the Zodiac.

The last likely Zodiac killing, of a San Francisco cab driver, takes place in October of 1969. But that isn't the end of the case. Two San Francisco police detectives, Dave Toschi (Mark Ruffalo) and Bill Armstrong (Anthony Edwards), keep pursuing leads as the years mount up — and so does the freelance enthusiast Graysmith, who becomes increasingly obsessed. There's no shortage of suspects, many self-admitted, most of them bogus. But as the evidence and strange indications proliferate, one man stands out — a bald, doughy child molester named Arthur Leigh Allen (John Carroll Lynch). Is he the Zodiac? Toschi and Armstrong definitely like him as a suspect — especially after a bizarre confrontation at the oil refinery where Allen works. But despite their most determined efforts, they can't make all the evidence fit. The movie's point of view — which is the point of view of Robert Graysmith — is that Allen is the man. And even though the Zodiac case was never solved, Fincher manages the considerable feat of bringing the picture to a satisfying narrative conclusion.

"Zodiac" bears little resemblance to Fincher's hyper-violent 1995 serial-killer hit "Se7en." This film is more ambitious and far more subtle — it's an essay in ambiguity. And while it's a long picture, it doesn't feel long at all. I wanted it to keep going for a while — just like the Zodiac case itself, which continues to percolate to this very day.

("Zodiac" is a Paramount Pictures release. Paramount and MTV are both subsidiaries of Viacom.)

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