A serial killer named Zodiac starts his trauma in the San Francisco Bay Area on 4th July, 1969 by shooting two people. In August, letters are sent to The San Francisco Examiner, The Chronicle and The Vallejo Times Herald. Each letter is part of a cryptic message that contained clues to identify the killer. On appearance of these letters, few investigators and reporters start the quest for the murderer. Prominent amongst them were homicide detective David Toschi (Mark Ruffalo) with his partner Inspector Bill Armstrong (Anthony Edwards), the expert reporter Paul Avery (Robert Downey Jr.) and a cartoonist Robert Graysmith (Jake Gyllenhaal). The four work in their own way to gather lots of circumstantial evidence against Zodiac. They get so much preoccupied by this case, that they almost forget their personal lives. But, Zodiac' salways a step ahead of them at any time. Graysmith continues his detective work and with the help of Avery, he's able to filter the suspects to one single person.