Assassino serial desafia detetives e jornalistas com charadas e mensagens codificadas, ameaçando mais assassinatos se demandas não forem atendidas. Corrida contra o tempo para desvendar sua identidade.
Detectives realize that a string of murders that predate the spree claimed by Zodiac appear to be connected and turn to them in hopes of finding clues in the investigation, perhaps also linking them to the #1 suspect, Arthur Leigh Allen.
A hoard of handwritten letters, kept in secret for decades, is revealed and offers a never-before-seen window into Allen's mindset and, more shockingly, incriminating details, which finally force the family to confront their suspicions.
