David Fincher's Zodiac was one of the highlights in a banner year for American cinema—not that audiences noticed. The $75-million picture grossed $33 million, a meager take that augurs poorly for its chances with that barometer of middlebrow consensus, the Oscars. It's doubtful that its appearance on a host of top-10 lists, or Tuesday's release of a special-edition director's cut DVD, will change mainstream opinion. Consider its various anomalies: It's a cop epic without a single shootout, a serial-killer flick in which all the blood is shed in the first act, and a taut procedural in which the case is never solved. In fact, it's one of the most unsatisfying thrillers you'll ever see—which is precisely how Fincher intended it.

Based on the true story of the murderer who terrorized the Bay Area in the late '60s and early '70s, Zodiac has been interpreted as many things: a meditation on obsession, a critique of the press, and an allegory of our information-glutted age. But often left unmentioned is what makes Zodiac truly—and sneakily—subversive: It's a Hollywood movie that champions due process. The movie meticulously depicts the exasperating rigamarole of a real-world investigation: Evidence and procedure trump gut feelings and brute force; suspects are hauled in and let go because the case isn't there. Zodiac emerges as a rebuke to the shoot-from-the-hip heroics of a show like 24. In a year of didactic, ripped-from-the-headlines movies, Zodiac's political critique was the most relevant of all, and hardly anybody noticed.

Zodiac's three acts follow an unusual trajectory. First come the killings. After a murder on the Fourth of July, 1969, the killer, who calls himself "Zodiac," writes a series of coded messages to the San Francisco Chronicle. Drawn to the case is Robert Graysmith (Jake Gyllenhaal), the paper's cartoonist and an introvert with a knack for puzzles and ciphers. He looms over the desk of Paul Avery (Robert Downey Jr.), the crime reporter, hungry for any morsel related to the murders. In the second act, we settle in for the methodical investigation by detectives Dave Toschi (Mark Ruffalo) and Bill Armstrong (Anthony Edwards). An informant gives them a promising lead, a weirdo by the name of Arthur Leigh Allen. But despite their dogged work, Toschi and Armstrong can't close the case—ballistics, fingerprints, and handwriting samples come back negative, seeming to exonerate their prime suspect.

The third and final act of the film takes place four years later, after the trail has gone cold. Into the void steps Graysmith, who is now writing a book about Zodiac. His digging turns up new evidence that points to Allen, but he still can't find a smoking gun. The movie ends with a coda showing one of Zodiac's surviving victims finally coming forward in 1991 and identifying a photo of Allen as the man who shot him. Title cards tell us that Allen died shortly before investigators could bring charges. The case was never solved and remains open in several counties where the Zodiac murders took place.

Ending on that inconclusive note, the movie may have alienated an audience conditioned to expect catharsis. It certainly was a different kind of movie from Fincher: In films like Se7en and Fight Club, the director left moviegoers dizzy with his operatic endings. Zodiac, much like the actual investigation, just peters out. The slick atmospherics and movie-star emoting of Se7en (also about a serial killer) give way to the painstaking work of building a prosecutable case.

The movie lavishes attention on the boring business of crime-fighting: getting a warrant, untangling jurisdiction, sifting through hundreds of tips. At first blush, Fincher's fastidious chronicling of the investigative process seems a clever expression of the theme of obsession (the movie's as obsessive as Graysmith!), or perhaps a sly poke at our bureaucratized culture. But it's more than that. Faithfully depicting the drudgery of workaday police life, the movie becomes a tribute to the law and the men who uphold it. There are no action sequences in Zodiac, but Toschi and Armstrong manage to make plugging away at thankless tasks without fuss seem heroic nevertheless.

Zodiac contrasts the work of the professional detectives with that of Graysmith, the amateur sleuth. The movie sympathizes with Graysmith's quest—but only to a point. At the end of the day, Fincher makes clear that connect-the-dots cleverness is not a substitute for expertise, and for procedure. What's more, Graysmith's inquiry demonstrates the danger of stepping outside legal boundaries. In a late scene, we see Graysmith conducting an interview with a woman whom he hopes will offer the missing piece he needs to solve the Zodiac case. Hollowed-out by the hunt, Graysmith just wants closure—and tries to put words in the mouth of the woman to get the information he's after:

In their final scene together, Graysmith delivers to Toschi a tour-de-force presentation of his theory that Allen is, in fact, their man. He marshals all the elements of the case against Allen: the same military-style boots that Zodiac was known to wear, the same size gloves and shoes, the tendency to misspell the same words, even a Zodiac-brand wristwatch with the same logo that the killer uses in his letters. But Toschi points out that while Graysmith's narrative sounds convincing, his case is built entirely on circumstantial evidence and wouldn't hold up in a court of law. Graysmith pleads with him to not think about the case like a cop. But Toschi cuts him off:

Graysmith isn't the only foil for Toschi and his deliberate style of crime-fighting. Fincher also implicates pop culture in his political critique, with the appearance of a different cop: Dirty Harry. After the Zodiac investigation winds down, Toschi attends the San Francisco premiere of Clint Eastwood's 1971 movie, which was based on the Zodiac killings. Dirty Harry is our fantasy of what a cop like Toschi should be. Confronted by bureaucratic thickets, the steely Harry sneers, "The law is crazy." His shoot-first-ask-questions-later style is condemned by his superiors, but is shown to be quick and effective. At the end of the film, Harry guns down the killer and throws his badge away—a final repudiation of a legal system that moves too slowly for him. Unencumbered by the law, Dirty Harry gave the audience the payoff the real world had denied. Toschi can't abide its vision of law enforcement and walks out of the movie:

Harry Callahan, it's worth noting, isn't the only movie cop connected to Toschi. Although it's only mentioned in passing in Zodiac, Toschi was also the model for Steve McQueen's character in the 1968 movie Bullitt. The San Francisco-set police drama featured one of the greatest car chases ever filmed and ended with Bullitt shooting the bad guy. Zodiac's Toschi isn't entirely immune to the appeal of glamour. His natty outfits and custom-made holster (which was copied by McQueen) attest to that. But in Fincher's film, he always remains grounded in the messy, mundane world of being a real detective.

Instead of Bullitt and Dirty Harry, Zodiac gives us Toschi and Armstrong, men who go about their business in a diligent, dignified manner. Their commitment to due process may not be sexy—it may even lead to the bad guy getting away—but the movie argues implicitly that sticking to that course is necessary. Like Toschi, Zodiac has little use for pop vigilantism and extra-legal tactics. A stoic, even moving, affirmation of the liberal society's values, Fincher's movie is a needed corrective in a world in which the invocation of Jack Bauer at a Republican debate wins loud applause.