In the late 1960s and early 1970s a serial killer who dubbed himself "the Zodiac" terrorised San Francisco. In a series of depraved letters to both the police and the press he claimed to have killed 37 people (although only seven were definitively linked to him). These letters typically began with "This is the Zodiac speaking..." and revealed a man who believed there was a brutal demon trapped within him that was collecting souls for the afterlife. Many of these letters contained messages hidden in ciphers that the police spent vast amounts of expert time attempting to decrypt. Of the four ciphers the Zodiac developed, only one was ever cracked.

In many ways, the Zodiac was the precursor to the modern serial killer stereotype - certainly that adopted by every second thriller. The big difference being that, in fiction, the good guys always outsmart and catch the serial killer, whereas investigators are no closer to catching the Zodiac today than they were 30 years ago.

The film Zodiac follows the investigations by the police and a group of reporters into the Zodiac killings and is fairly heavily based on fact (specifically the numerous Robert Graysmith books on the Zodiac). Unsurprisingly, the correspondence from the Zodiac had a profound effect on the police and reporters that it was sent to. Director Fincher, whose Seven was obviously influenced by the style of the Zodiac killer, returns to the serial killer theme with Zodiac but approaches it with a very different style to past outings. Zodiac is much more a macabre drama than a thriller. Check the back seat before driving home after this one!