Based on the terrifying true story, the film chronicles the desperate hunt for one of America's most notorious serial killers, as seen through the eyes of a police detective (Chambers) and his son (Culkin) who become obsessed with the murders and endanger their family in the process.

On Dec. 20, 1968, a teenage couple is viciously murdered at a desolate lover's lane sending a wave of shock, horror and confusion through the small town of Vallejo in Northern, California. Sergeant Matt Parish (Chambers), husband to Laura (Tunney) and father to his twelve-year-old son, Johnny (Culkin), takes on the investigation. With very few leads to go on, the pressure mounts as a renegade local newspaper reporter sensationalizes the tragedy, spreading fear in the once quiet community. At home, Parish tries to come to terms with the dramatic emotional effects the horrible murders and the failing investigation have brought on his family.

Six months later, July 4th, 1969, the Zodiac strikes again. Amidst the festive summer holiday, a young couple is gunned down at close range in a dark parking lot. Then, the unthinkable happens. A taunting phone call to the police is made within an hour of the attack confessing to the murders. A letter is sent to Bay Area newspapers threatening to kill a dozen more. Included in this letter is a cryptogram, which according to the Zodiac, if deciphered, would reveal his identity. Determined to end the madness and terror, Matt descends into the all-consuming complexity of the case: codes, cyphers, composite sketches, psychological profiles, and bizarre astrological patterns. His desperate obsession, fueled by the increasing media coverage, sends he and his family to the edge of ruin.

After another savage double murder in broad daylight, Parish believes he has finally identified a potential suspect. This is his one shot. Parish has everything to lose and everything to gain if he solves the mystery of the Zodiac and captures the infamous serial killer.