Ask baby boomers about the year 1969 and most will remember Neil Armstrong's "one small step." It was the summer of Woodstock, and of the bizarre murder of actress Sharon Tate. 1969 is also memorable as the year the death toll in Vietnam outstripped that of the Korean War. In San Francisco, 1969 was the year of the Zodiac killer. Among the serial murderer's many letters to the press, one included the threat that he would hijack a school bus and massacre the children. In 1969, David Fincher (Panic Room, Seven, Fight Club), the director of Zodiac, was a seven-year-old San Francisco schoolboy.

Perhaps because of that vivid memory, Fincher's direction is extraordinary. Equally impressive is the screenplay by James Vanderbilt, who drew from Robert Graysmith's entertaining and encyclopedic books about the Zodiac killer. For the director, the screenwriter and the author, the tale is as much a human-interest story as it is a thriller.

In the film, the events of 1969, and of the decades that followed, unfold through the eyes of Graysmith (Jake Gyllenhaal), then a cartoonist for the San Francisco Chronicle. Graysmith liked puzzles, and the Zodiac killer's first letter to the press was written in code. Hooked on the pursuit, he befriends the Chronicle's troubled crime reporter Paul Avery (Robert Downey, Jr.), and begins to collect all the information he can find on the serial murderer. Graysmith stayed with the case the longest, and so was the screenwriter's logical choice for a main character, but Inspector David Toschi (Mark Ruffalo), the San Francisco Police Department detective who headed the Zodiac investigation, was clearly a source of inspiration.

Long after his partner moved on to another job at the department, Toschi stayed with the investigation, although it was deemed a "cold case" in 1974. In 1978, the detective became the subject of an Internal Affairs probe. He was declared innocent of the charge that he fabricated a letter from the Zodiac killer, but he was not entirely vindicated--at least not until now. Heralded for inspiring many onscreen cops, such as Steve McQueen's character in Bullitt and Clint Eastwood's in Dirty Harry, Toschi was interviewed by the filmmakers, along with Graysmith, and the two became technical consultants for the movie.

In Zodiac, and in Graysmith's two books, Toschi identifies the murderer early on. Later, Graysmith's independent research leads him to the same suspect, Arthur Leigh Allen (John Carroll Lynch), a convicted pedophile. Allen was never arrested because the San Francisco D.A. did not think the case against him was strong enough: All the evidence was circumstantial. Zodiac is the incredible story of Graysmith and Toschi, and the other men and women whose lives became intertwined with the serial killer, the survivors haunted by their encounter with him, and the policemen from four cities whose careers were dominated by their investigation into the murders.

Vanderbilt's screenplay is crammed with facts and, at first, it is easy to get caught up in them, but as the movie progresses, the human drama--Graysmith's obsession with the case, Avery's debilitating drug addictions and Toschi's frustration--propels the narrative, and it's riveting, all 154 minutes of it. Editor Angus Wall (Panic Room) deserves Academy consideration for his work, which maintains a thriller's pace while never sacrificing the characters' emotions to breakneck cutting. Zodiac was shot with a high-definition video camera, and to the trained eye it looks different than movies shot on 35mm. It is brightly lit, and the image looks flatter, but that lends an appropriate vintage feel to Zodiac which, in design and sensibility, must straddle a gap of nearly 40 years.

Fincher never loses control of the film or of the cast, keeping his audience squarely focused on the plight of his characters. Ruffalo's Toschi is reminiscent of his In the Cut character, Detective Malloy, but this time the actor has a script. Ruffalo sets new standards for restraint as the cover for a complicated personality, lending veracity to a character that could easily become too heroic for the narrative. Gyllenhaal is excellent as the reserved cartoonist turned obsessive, indefatigable researcher. Lynch's (Gothika) Allen is menacing, constrained and confident, the portrait of a serial killer. Unfortunately, Fincher has him stress the "latent homosexual" profile attached to Allen at the time, which reflects an all-too-common stereotype linking pedophilia and homosexuality.

Zodiac is about obsession, and about the intriguing consequences of serendipitous collaboration: First, there is the team of Graysmith, a dilettante, and Avery, a professional journalist, and then Graysmith, an aspiring author, and Toschi, an experienced investigator. While skilled handwriting analysts ferret through Zodiac's letters for clues, an ordinary couple breaks the killer's code. A cartoonist becomes an author, and a great detective is defamed by a hoax. A cold case is reopened. It's a story the best fiction writer would be hard-pressed to invent, and it's a movie that could have followed the well-worn path of lionizing its protagonists. Fincher, however, meticulously avoids cinematic hyperbole, which is what gives Zodiac such uncommon authenticity.