Karen McCann is happily married to Mack and has two daughters, 17-year-old Julie (from Karen's previous marriage) and six-year-old Megan (with Mack). She lives in a lovely two-story home in Pacific Palisades, California and has a good job in a museum.
Karen's perfect world is suddenly shattered when Julie is violently raped and murdered while Karen listens helplessly on the phone from her car. Detective Sergeant Joe Denillo assures Karen there is enough DNA evidence to find and convict the killer. He encourages the McCanns to seek counseling.
At a support group, they meet people in similar circumstances, including Albert and Regina Gratz, and Sidney Hughes. During the meeting, Karen overhears Albert talking to Sidney about something which alarms Regina.
The DNA tests reveal the killer, Robert Doob, a delivery man with a criminal record. At the trial it is clear Doob is guilty, but because the defense did not receive a sample of the DNA evidence, the judge dismisses the case. Karen and Mack are dumbstruck as Doob walks free. When Doob launches an antagonizing apology at Karen it causes Mack to snap and furiously attack Doob, but is overpowered by guards and Doob walks out unharmed.
Mack is desperate to return to a normal life, but Karen cannot stop thinking of Doob. She finds the apartment where he lives, then keeps detailed records of his movements, stalking him. After observing Doob urinate on a customer's lawn after a delivery she goes to Denillo — but he tells her there's no evidence of intent. Karen attempts to warn the delivery customer, but the woman only speaks Spanish and does not understand her.
Karen later learns that the murderer of the son of a member of her support group has been killed in a drive-by shooting, just days after being released from prison. Angel, also in the self-help group, tells Karen the best way to get over her grief is to focus on having good experiences with her living daughter — and Karen realizes she has been so fixated on Doob that Megan has been deprived of her attention. Meanwhile, Doob has gone to Megan's school and struck up a conversation with the girl during recess. When Karen comes to pick up Megan, Doob deliberately intimidates her.
Worried for Megan's safety, Karen's sanity is on the rocks and remembers what happened to the killer of her friend's son and approaches Sidney, who admits the drive-by shooting was set up by him and Martin. Karen demands their help and they agree to find a weapon, train her, and plan the murder, but tell her she has to carry it out. Karen agrees and they begin plotting. She also joins a self-defense class which helps her gain more confidence, helps rekindle her sex life with Mack, and improves her relationship with Megan. Karen feels encouraged. Although Martin doubts Karen is capable of murder, Sidney gives her a gun.
The next day, Angel reveals that she is really an undercover FBI Agent investigating vigilante activity. Angel warns Karen not to kill Doob. Karen calls Sidney to tell him she cannot go through with it. However, she soon changes her mind when she learns that the Hispanic customer she tried to warn about Doob has been raped and murdered just as Julie had been. Karen is so furious she accuses Denillo of not finding enough evidence, letting Doob go free. Hearing Doob has again gotten off on a technicality bolsters her resolve. Karen decides that the only way to avenge her daughter's murder and save her family from Doob is to kill him.
She sets a trap to lure Doob into her home, so that she can say killing him was self-defense. It works. She shoots Doob dead and calls the police. Denillo arrives and tells Karen that he knows the truth and that she hasn't fooled him, to which she replies, "Prove it." He decides to tell his colleague that it was a "clear case of self-defense". When her husband arrives, he sits beside her, holding her hand, knowing what she has done.
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