Zodiac is based on real events surrounding a notorious serial killer -- known only as the Zodiac -- who terrorized the San Francisco Bay area in the late 1960s and early 1970s. It's an intense and chilling drama.

Shortly after midnight on July 5th, 1969, police in Vallejo, California, received an anonymous call from a man who said he had committed a double murder moments before in an empty parking lot. Darlene Ferrin and Michael Mageau had driven down a narrow access road leading to the Blue Rock Springs Golf Course just prior to midnight. They parked and talked for some time before noticing another car had pulled up next to them. The driver suddenly sped off only to return moments later. He then stopped his vehicle behind them, blocking the only way out of the lot. The man stepped out of his car holding a flashlight. He shined it into the passenger side window and shot the couple several times. Mageau miraculously survived. Ferrin, however, did not.

The caller also told police he committed the murders of Betty Lou Harris and David Faraday on December 20, 1968. The two were on their first date when they decided to stop in a secluded area. A nearby resident later found them both shot to death.

The killer, identifying himself as Zodiac, mailed three separate letters to the editors of the San Francisco Chronicle, San Francisco Examiner and Vallejo-Times Herald. Each envelope arrived on August 1, 1969. The letters gave explicit details about both murders -- and also enclosed were three baffling cryptograms. Zodiac insisted the letters and bizarre ciphers be printed in full on the front page of the late edition of each paper or he would kill 12 more people during the week after. The killer said the secret codes would unlock facts about who he was.

Paul Avery (Robert Downey Jr.) is the San Francisco Chronicle’s crime reporter singled out by the Zodiac as the one to receive his letters. When Avery offends the killer in one of his articles, the murderer threatens him, thereby forcing the reporter to carry a gun wherever he goes.

Now that a history teacher has cracked a part of the Zodiac’s ciphers, editorial cartoonist Robert Graysmith (Jake Gyllenhaal) has become obsessed with finding the hidden meaning behind the code. He starts piecing together information about the killer to write a book concerning the case, hoping to help discover the serial killer’s identity.

The Zodiac suddenly changes his modus operandi and kills a taxi driver, narrowly evading capture. Detectives David Tosco (Mark Ruffalo) and Bill Armstrong (Anthony Edwards) are brought in to dig deeper into the confusing Zodiac investigation.

Downey Jr. stands out here for his portrayal of Avery, a gifted reporter who becomes a recluse as he spirals into alcohol and drug addiction. Gyllenhaal also delivers a strong performance as Graysmith, who wrote the book on which this film is based. Finally, director David Fincher deserves kudos for his skillful execution of the film and for taking full advantage of the talents of his stellar ensemble cast.

(Released by Paramount Pictures and rated “R” for some strong killings, language, drug material and brief sexual images.)