The book describes the investigations of the many law enforcement branches such as the LAPD, the FBI, the CIA, etc., that worked on the case and other murders that the "Zodiac" had proclaimed he committed, including the 1966 Cheri Jo Bates stabbing.

Later chapters deal with Graysmith's many theories on the case, and the book eventually cites two possible suspects (who are given pseudonyms), Bob Starr and Donald Andrews, and details some of the circumstantial evidence against them.

[2] Although the case was never solved, there were instances where solid evidence was held among a number of suspects, even after the Zodiac murders had stopped.

Robert Graysmith was a cartoonist working for The San Francisco Chronicle in the 1960s when the Zodiac murders started.

The film follows the work of Graysmith as a cartoonist in 1969 aiding the authorities in figuring out the identity of the infamous Zodiac killer.