A serial killer in the San Francisco Bay Area taunts police with his letters and cryptic messages. We follow the investigators and reporters in this lightly fictionalized account of the true 1970's case as they search for the murderer, becoming obsessed with the case. Based on Robert Graysmith's book, the movie's focus is the lives and careers of the detectives and newspaper people.
Robert Graysmith is a cartoonist who works for the San Francisco Chronicle. His quirky ways irritate Paul Avery, a reporter whose drinking gets in the way of doing his job. The two become friends thanks to a shared interest: the Zodiac killer. Graysmith steadily becomes obsessed with the case, as Avery's life spirals into drunken oblivion. Graysmith's amateur sleuthing puts him onto the path of David Toschi, a police inspector who has thus far failed to catch his man; Sherwood Morrill, a handwriting expert; Linda del Buono, a convict who knew one of the Zodiac's victims; and others. Graysmith's job, his wife and his children all become unimportant next to the one thing that really matters: catching the Zodiac.