On July 4, 1969, an unknown assassin shoots a couple in love on the outskirts of Vallejo, California. A month later, the publishers of major newspapers begin to receive encrypted letters in which the killer, calling himself Zodiac, threatens to kill 12 more people if the text of the letter, containing the secret of his identity, is not published on the front page. In addition to police officers from different states, the investigation involves a seasoned San Francisco Chronicle reporter Paul Avery and a young cartoonist Robert Graysmith, who works at the same newspaper. Soon the killer strikes again.