rated R

“Zodiac” seems like an easy sell: it’s based on the true story of a serial killer who terrorized San Francisco and northern California in the late 1960s, and it’s directed by David Fincher, the auteur behind “Seven” and “Fight Club.” It should be a recipe for suspenseful, high velocity filmmaking. Instead, it’s a quietly confident film, a gripping police procedural and psychological drama that eschews serial killer clichés and grisly violence in favor of a tight focus on a trio of compelling characters.

As “Zodiac” begins, the titular killer claims his second and third victims, a couple parked on a secluded lover’s lane. Eager to taunt police and the press, Zodiac begins sending letters to the police and San Francisco’s three major newspapers, taking credit for the murders and promising more mayhem in the future. Jake Gyllenhall stars as Robert Graysmith, a political cartoonist at the San Francisco Chronicle who becomes obsessed with the case, especially the ciphers that Zodiac includes in his letters. Graysmith befriends crime reporter Paul Avery (Robert Downey Jr.), and the two soon spend their spare time drinking at a local dive and swapping Zodiac theories. Meanwhile, police detective Dave Toschi (Mark Ruffalo) also doggedly pursues the case, giving himself solely to the pursuit of Zodiac.

Most of “Zodiac” concerns the police investigation of the crimes. The murders, which Fincher films with an unflinching intensity, are placed at the front of the film, leaving plenty of room for Toschi, Avery and Graysmith to obsess for the rest of the film. Finding a cast better suited for the brooding obsession required for “Zodiac” would be difficult. Gyllenhall and Downey both play to their strengths. Gyllenhall, who gets his chance to shine at the end of the film, gives Graysmith a kind of nerdy, but endearing, awkwardness, and Downey essentially acts like himself—arrogant, rude, slightly fey, but likeable in a roguish sort of way. Gyllenhall doesn’t get a chance to shine until the end, though, and it’s Ruffalo who carries most of the film. Toschi is quiet yet tenacious, a quirky cop who immerses himself in the case without question. And though they never actually meet up with the killer, all three ultimately walk away with battle scars: Graysmith’s obsession nearly ends his marriage, Avery drowns himself in booze and drugs, and Toschi gets tossed from the case after he’s accused of writing a fake Zodiac letter.

“Zodiac” works because Fincher and screenwriter James Vanderbilt don’t try to shoehorn the story into the serial killer movie narrative popularized by “Silence of the Lambs.” There are no frantic police chases across the city, no SWAT teams or heroic cops swooping in at the last second to save a potential victim.

Instead, “Zodiac” plays out like a real-life police procedural—there are false leads and phony confessions, a lack of solid, useable evidence and periods of days, months, and even years when all the leads go cold and nothing happens. It doesn’t sound like the sort of thing that would lend itself well to film, but Fincher makes it work. Time passes, characters get older and do other things in their lives, but they remain obsessed with the Zodiac case. Fincher documents the story painstakingly—there will be a brief scene showing the arrival of the latest letter from Zodiac, and then a title card informs the viewer that another year and a half has passed before something else happens. That Fincher can do this and still maintain a sense of mood and urgency throughout the film is remarkable.

“Zodiac” also has an unerring sense of place and time. From the opening shot of the San Francisco bay alit with fireworks to a scene set inside a screening of “Dirty Harry” (which drew inspiration from the Zodiac case) to Brian Cox’s scene-stealing performance as celebrity attorney Melvin Belli, “Zodiac” is grounded in the California of the mid-1970s.

“Zodiac” is about a half-hour too long, but the story is so strong, and the characters so enjoyable to watch, that it’s hard to say what should have been left on the cutting room floor. A long sequence involving Toschi’s dogged pursuit of a false lead on a suspect who almost fits the bill takes up most of the film’s second hour, but ultimately dead ends. By the time the police investigation has died down and Robert Graysmith picks up the Zodiac torch, the film is reinvigorated, with more suspense and a stronger narrative drive. It’s a good move, but one that comes at the end of the film, about 40 minutes too late.

That’s what real police work is like, though, especially in the case of the Zodiac. By the time the credits roll, there’s no real answer to who the killer was or why he did it. There are only more questions and a trail of people whose lives have been irrevocably altered, sometimes for good, sometimes for bad. It’s not the sort of ending Hollywood, or moviegoers, typically crave, but in the case of “Zodiac,” it’s frustratingly perfect.