This is based on the true story of a serial killer in the 1970’s in California who highlighted his murders with cryptic letters to newspapers. He killed at random, and called himself The Zodiac, and wrote his letters in a code. He also often sent bloodstained items of clothes along with these letters as a sort of proof. He was never caught. Robert Graysmith was working as a cartoonist at one of the newspapers where the letters arrived, and became caught up in the case. The film is based on his book of the events of the time.