Dr. Samantha "Sam" Waters (Ally Walker) is a forensic psychologist working for the FBI's (fictitious) Violent Crimes Task Force ("VCTF"), based in Atlanta, Georgia.

Sam performs all of her duties diligently and competently, her drive coming from experiencing both a professional and personal tragedy years earlier in which her husband was murdered by a serial killer known only as "Jack of All Trades."

Due to Jack's dangerous and pathological nature, Sam must live under 24/7 police guard in a former firefighter station with her seven-year-old daughter Chloe (Caitlin Wachs, later Evan Rachel Wood); and her best friend, artist Angel Brown (Erica Gimpel).

After helping her mentor Bailey Malone (Robert Davi) on a difficult case, Sam comes out of retirement and reclusiveness to join the FBI Violent Crimes Task Force.

Also on the team are detectives John Grant (Julian McMahon) and Nathan Brubaker (Michael Whaley), computer hacker George Fraley (Peter Frechette), forensic pathologist Grace Álvarez (Roma Maffia).

To mislead and tease the viewers, Albert Newquay, the real Jack's true name, was first mentioned in season two when he took refuge at the Maryland home of his wealthy mother Miriam (Louise Fletcher) to recover after being shot by Sam.

Newquay then appeared on-camera for the first time in season three posing in disguise as Ed Post, the seemingly buffoonish sheriff of a sinister fictional small northern California town.

Rachel struggles to help her self-destructive younger brother Danny (Raphael Sbarge); Grace faces dealing with her second pregnancy after her husband leaves her; and George develops an addiction to painkillers after being injured in a minor car accident.