In the 1970s, a killer who called himself "Zodiac" appeared in San Francisco. After killing people, he sent a letter to the media, leaving a code and clues to provoke the police. Many mistakes made the case deadlocked, and the police gradually became helpless. The serial killings also attracted the attention of Robert (Jack Granhall) and Paul (Robert Downey Jr.), reporters of the San Francisco Chronicle. With the help of policeman David (Mark Pfalo), they began to investigate this series of serial murders. While fighting the "Zodiac" killer, their personal lives also faced great challenges.