Paul Kersey has managed to recover from his shattered life and moved on, and is now dating a Los Angeles radio reporter, Geri Nichols. They go to pick up Paul's daughter, Carol, from the mental hospital. They spend the afternoon at a fairground, where Paul's wallet is stolen by a gang, consisting of Nirvana, Punkcut, Stomper, Cutter, and Jiver. The gang splits up when Paul chases them; he goes to pursue Jiver, whom he corners in an alley but lets the hood go after Jiver tells Paul that he does not have the wallet. The gang finds Paul's home address in his wallet and later breaks into his house. They gag and restrain Rosario, Paul's housekeeper, and begin taking turns raping her. When Paul arrives home with his daughter, he is beaten unconscious. Rosario tries to call the police, but Nirvana kills her with his crowbar. They kidnap Carol and take her to their hideout, where one of the gang members rapes her. Carol attempts to flee by running through a plate-glass window, falls onto an iron fence, and is fatally impaled. When the police arrive, Lieutenant Mankewicz asks for help identifying the muggers, but Paul refuses. After Carol's funeral, he takes his Beretta Model 84 handgun to a low-rent inner-city hotel as a base of operations. The next evening, he observes and follows Stomper into an abandoned building as a drug deal is about to be made. Paul shoots twice one of the dealers and orders the others out before he shoots Stomper twice. The following night, he hears screams from a couple being assaulted by four muggers, which includes Jiver, in a parking garage. Paul kills two rapists and wounds Jiver. Then Paul follows Jiver's blood trail into an abandoned warehouse and kills him.
The LAPD suspect that the murders are the work of a vigilante and ask the NYPD for guidance as they dealt with a vigilante spree years earlier. NYPD Detective Frank Ochoa suspects it may be Paul again and is sent to assist with the case. Ochoa fears that Paul, when caught, will reveal that he was released without being charged for killing the nine muggers in New York City. Ochoa meets with Mankewicz, who suspects that Ochoa is not telling him everything. Ochoa intrudes into Geri's apartment and tells her about Paul's previous vigilante killing spree in New York City. After Paul returns to his house, Geri confronts him about Ochoa's revelation, but he denies it. Ochoa follows Paul to a local square, where Paul is tailing the three remaining gang members. He follows them to an abandoned park, where a major arms and drug deal is underway. A sniper scouts Paul and attempts to kill him, but Ochoa warns Paul and fatally shoots the sniper. Ochoa is mortally wounded by Nirvana, and Paul shoots Cutter in the head and another dealer and wounds Punkcut. The arms dealer tries to get away, but Paul shoots him and causes his car to drive off a cliff and explodes and Nirvana escapes. Ochoa tells Paul to avenge him before he dies. Paul escapes, and Punkcut dies from his injuries after giving information about Nirvana to the police.
Paul learns from one of Geri's colleagues that the police are preparing a tactical unit to capture Nirvana. He obtains a police scanner and, by monitoring police radio traffic, finds out when and where the arrest is going to take place. He drives to the location to kill him, but Nirvana, under the influence of PCP, slashes his arm and stabs a few officers while he tries to escape. Tried and found criminally insane, he is sent to a mental institution. Geri is writing a story about the case and capital punishment and takes Paul to the hospital to meet the doctor treating Nirvana. While there, Paul steals another doctor's identification card and uses it to enter the asylum and confront Nirvana. After repeatedly stabbing Paul with a shiv, Nirvana ends up plunging his arm into a high-voltage device, which Paul turns on, electrocuting him to death. Donald Kay, a sympathetic orderly at the hospital, recognizes Paul from the newspaper coverage of Carol's death, and gives him three minutes to escape before he rings the alarm. Geri goes to Paul's house, where she finds out that he made a fake doctor's ID. Upon hearing a news report of Nirvana's death on the radio, she realizes that Paul really is the vigilante that Ochoa claimed him to be. She takes off her engagement ring and leaves him, with Paul arriving moments later.
A few months later, Paul speaks about a new architectural design. He is invited by his employer to a party. When Paul is asked if he is able to attend, he answers, "What else would I be doing?", Paul then walks off into the night as the film ends.
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