David Kirmani,a medical professional, lives in an apartment with his sweetheart, Erica Bain, a radio host, and his dog. They usually take the dog out for a stroll in nearby Central Park and let him run and fetch. One day while at the park they let the dog run without a leash, and when he does not return or respond to their calls, they frantically search for him. They eventually find him being held by three men who want a reward. When David refuses, they start to molest Erica and David intervenes. Two of the men assault the couple, while the third uses a camcorder to film this incident. Erica is knocked unconscious, and regains her senses several days later in a hospital. She is told the shocking news that David was killed and the dog is missing. A traumatized Erica returns home to try and regain her life. She also visits the police station but does not get much help. Fearful of a repeat of this type of incident, she gets an unlicensed gun and carries it with her all the time. One day she witnesses some youths terrorizing subway passengers, forcing them to get off at the next station. When they approach her, she shoots them dead. She uses her gun several more times, always gunning down people committing violent crimes. One day she is summoned to the police station to identify a man who may have been responsible for David’s death and the assault on herself. She does recognize him, but refuses to identify him. Subsequently she finds out his whereabouts and plans to avenge her lover’s death, not realizing that Detective Mercer has her as a suspect in the recent killings and is set to apprehend her. The question remains: will Erica be able to avenge David’s death and her attack before being arrested herself?
Radio host Erica Bain (Jodie Foster) is working and living a wonderful life in New York City without a care in the world. One night she and her fiancé, David (Naveen Andrews), are brutally attacked by three thugs in the park while being recorded by one of them. David is killed and Erica is beaten within an inch of her life. Erica is traumatized to a point where she can not sleep and will not leave her apartment. To cope and work past this tragic event, Erica becomes someone else. She buys a gun and begins to roam the streets at night looking to take her revenge on the men who were responsible for killing her one true love. One late night, Erica is in the back of a convenience store about to purchase a soft drink when a man comes in and shoots the clerk, who is his estranged wife. He realises Erica is a witness and comes after her but she shoots him and escapes, taking the store CCTV video with her to hide her involvement. Days later, Erica is on the subway when she witnesses an attack on a youth by two men. They steal his iPod, and when an old man intervenes they threaten him and his son. The three of them hastily leave the train at the next stop and the two men approach Erica as she is the only other person in the carriage. They pull a knife on her but she shoots them both and quickly leaves. The City of New York is fascinated with the exploits of this vigilante killer, and NYPD Detective Mercer (Terrence Howard) is assigned to catch this person, who they believe is a man, before he kills again. Erica interviews Mercer as part of her radio show and they become friends, both linked by their inability to sleep at night. He tells her that he is prepared to go beyond the law to catch some criminals, but asks Erica to erase this comment from her tape. Erica continues to kill, rescuing a drugged prostitute from a car but, in killing the driver as he tries to run them over, the prostitute breaks her leg and when the police arrive, take her to hospital, though she is very close to death. Later on Erica gets a lead of where she can find the three men in the park and goes there to seek vengeance. She manages to kill all but the last one, being then saved by Detective Mercer whom she had already befriended and let her kill the last thug using the Mercer's gun so no one would link her to the killings.