The first rule about “Zodiac” is simple: You do not talk about “Fight Club.” Or, for that matter, any other movie directed by David Fincher (“Panic Room,” “Se7en.”) Why? Well, this film’s sort of different. Famous for being a typical Virgo — meaning he’s a perfectionist who pays fanatical attention to detail — Fincher sometimes catches flack for over-directing his movies. And while some of that perfectionism may linger in “Zodiac,” the director’s latest film only benefits from his painstaking efforts to get everything right.

Indeed, it’s this obsession with detail that makes “Zodiac” an almost perfect example of what a detective film should be. Engrossing, eerie and — despite its near-three-hour length — concise, it overcomes the difficult task of pulling viewers into an unsolved, and perhaps unsolvable, mystery. As the film’s three main characters, played with depth and charisma by Jake Gyllenhaal, Mark Ruffalo and Robert Downey, Jr., become obsessed with discovering the Zodiac killer’s identity, their pain and frustration create scenes of enthralling tension, rather than unwelcome discomfort.

Based on the real-life cold case of a San Francisco serial killer known only as Zodiac, the film opens with what is believed to be the first crime — a young couple is gunned down in their car late at night. Next, a letter arrives at the editor’s desk at the Chronicle, allegedly written by the killer and requesting that a cipher be published in the next edition of the newspaper. This cipher catches the eye of cartoonist Robert Graysmith (Gyllenhaal), whose detailed research on the case eventually comprises the real-life book called, you guessed it, “Zodiac.”

As more gruesome murders and coded letters turn up, the case gains national attention and causes a media uproar (think Anna Nicole, except more killing and less cleavage). The killer becomes a sort of cultural icon — he makes the front page, inspires paraphernalia and even gets a TV appearance (via telephone, of course) with local celebrity Melvin Belli, played like a pro by Brian Cox. But as the body count mounts, so does the evidence, and sorting it all out becomes a Herculean task for which human minds prove tragically ill-equipped.

Though the material is heavy, “Zodiac” refuses to get weighed down, and instead remains light on its feet thanks to a keen awareness of irony. When the press and the police should be working together to save lives, they are at each other’s throats. When the evidence turns up a convincing suspect, no arrest is made because bureaucratic technicalities will not allow it. The case is a no-win situation for anyone who dares to investigate it, yet the man who comes closest to solving it is a cartoonist who learns Detective Work 101 by checking out books from the public library.

Even more refreshing is that “Zodiac” manages, at times, to be laugh-out-loud funny. Robert Downey, Jr., adding an insubordinate air to his alcoholic news reporter Paul Avery, takes the character up a notch in the same way Johnny Depp personalized Captain Jack Sparrow. And Gyllenhaal’s juvenescence is the magic that makes even Graysmith’s most mundane actions sparkle. While it may be a wild-goose chase, though, “Zodiac” is by no means all fun and games — that is, unless it’s considered fun to watch a woman being stabbed to death, or to nearly wet oneself during a “Silence of the Lambs”-inspired dark basement scene. And it’s no game to watch the personal lives of the characters deteriorate and to then have the thought, “This really happened. These men cared that much about finding the killer, and they never got closure.”

But — and you’ll want to know — does the audience? Let’s just say that “Zodiac” tidily completes a very large circle that viewers, no matter what their sign, will be glad to accept as an end. And no, it has nothing to do with multiple personality disorder.