The movie is based on real events and tells the story of the investigation of a serial killer nicknamed Zodiac, who committed 37 murders in San Francisco in 12 years.

The events of the film begin on July 4, 1969, when Zodiac committed his second attack. On August 1, the killer sent three cipher letters to major newspapers, including the San Francisco Chronicle. A man calling himself Zodiac reported that the killings would continue… Two of the paper’s staffers, Paul Avery, who writes about crime, and cartoonist Robert Graysmith, were interested in his messages.