When police officer Jack Murphy sees teenager Arabella McGee steal his car, he chases after it. After crashing into a pizza parlor, Arabella takes off on foot. Murphy chases her into an alley, where she assaults Murphy with vulgar language, kicks him in the groin, and disappears. The next day, at a crime scene, he and his partner, Art Penney, investigate the death of a girl associated with drug kingpin Frank Vincenzo. Nearby in the dirt, Murphy discovers a gold locket inscribed with the name “Anthony Alberto Vincenzo.” At a restaurant, Murphy announces to Frank Vincenzo that he is going to arrest Frank’s brother, Anthony “Tony,” for murder, and Frank should persuade Tony to turn himself in. Instead, Frank comments that Murphy might be subject to “Murphy’s Law,” which posits that anything that can go wrong will go wrong. Frank Vincenzo suggests that Murphy could be run over by a truck, his gas heater could blow up, or his tire could fall off on the freeway. Murphy replies that the only law to which he gives any credence is: “Don’t f__k with Jack Murphy.” Elsewhere, a former convict named Joan Freeman arrives by bus and meets Mr. Cameron, a private detective. Cameron demands more money for the information Joan requested because one individual, Ben Wilcove, lives in a remote area and was hard to find. When Joan refuses to pay, Cameron tries to blackmail her. She finally agrees to his fee, but after snatching the envelope from his hand, she shoots him in the throat. Later, Joan calls Murphy, announces that she is going to kill him, and hangs up. The next morning, Joan takes photographs of Murphy walking to his car. Soon, Murphy and his partner Art Penney follow a lead to the airport to arrest Tony Vincenzo before he leaves for Las Vegas, Nevada. During a subsequent police pursuit, Murphy kills Tony. Later, at Tony’s funeral, Frank promises his mother that he will avenge Tony’s death. At Madame Tong’s strip club, Murphy watches his former wife, Jan, do a striptease. After her performance, he questions why she wants to earn money in such a seedy establishment. She claims to be legitimately pursuing her career as a dancer, but he argues she is wasting her time. Murphy follows Jan as she drives home with Carl, the club owner. He sips liquor from his flask as he watches them go inside Jan’s apartment building. As Murphy drives away, he is followed. The next day, as Murphy buys a bottle of liquor, he sees Arabella and chases her into a public restroom. As he arrests her, obscenities flow from her mouth. That night, Murphy returns to Madame Tong’s, tells Jan she looks like a whore on stage, and leaves. When he gets in his car, Joan emerges from the back seat and knocks him unconscious, She follows Jan and her boyfriend in Murphy’s car, murders them in front of Jan’s apartment building, and leaves Murphy parked in front of his house. Jan’s neighbor gives police a description of Murphy’s car and license plate, and Murphy is arrested. During interrogation, officers claim he killed Jan and her lover in a jealous rage, but Murphy insists he was framed. Meanwhile, a ballistics report identifies the murder weapon as Murphy’s gun. He is taken to a holding cell, and handcuffed to Arabella. They trade insults until she punches him, and a deputy breaks up their fight. Murphy grabs the deputy’s gun, knocks him unconscious, and uses Arabella as a human shield to escape. On the roof, Murphy steals a police helicopter, but soon discovers the fuel tank is almost empty. He lands the helicopter on the roof of a barn, which collapses. As Murphy and Arabella emerge from the wreckage, three men surround them, and Arabella requests they call the police. When Murphy comments that the men run a "dope farm" and have no intention of calling police, they knock him unconscious, but he comes to, steals a machine gun, and saves Arabella from being raped. They steal a truck, and take shelter at a cabin in the woods, where, as his adrenalin wears off, Murphy loses consciousness from his wounds. However, Arabella finds keys to unlock her handcuffs. When cabin owner Ben Wilcove appears and asks who she is, Arabella explains that his friend, Murphy, is hurt and needs help. Ben, a former army medic, stitches Murphy’s wound, and diagnoses a concussion. In the morning, Murphy and Arabella fight and she disappears. Later, Murphy borrows Ben’s old service revolver and leaves in the stolen truck. Soon, Ben senses he is not alone. Joan takes a rifle from Ben’s gun rack, and without a word, kills him. As Murphy drives toward the city, he sees Arabella and orders her into the truck. At a rest stop, Arabella shows Murphy a front-page story in the newspaper, accusing him of Ben’s murder. Murphy blames Frank Vincenzo for the string of deaths and demands that Arabella leave, but she refuses because she has been named as an accomplice in Ben’s death, and police are also looking for her. In the city, as Joan dines at an expensive restaurant, another diner, Judge Kellerman, approaches and asks if they have ever met. She sidesteps the question and invites him to join her for dessert. Later, at a hotel, Joan gives the judge a bath, and kills him by drowning and electrocution. Meanwhile, Murphy and Arabella sneak into Frank Vincenzo’s private penthouse, and Murphy accuses Frank of the murders of Jan, Carl, and Ben, but he denies any involvement. When Murphy plays “Russian roulette” with Frank, the gangster is reduced to tears, and Arabella senses that he is telling the truth. Murphy persuades Art Penney to search police records for old cases on which he and Ben worked. Of the three recently released suspects Penney uncovers, Joan Freeman is the most logical choice, and she is living in downtown Los Angeles at the Sunset Hotel. As Murphy and Arabella head to the hotel, Joan strangles her probation officer. Murphy searches Joan’s apartment and discovers her photo album with several pictures of him, Madame Tong’s, and Ben Wilcove. Arabella opens a closet and out falls the probation officer’s body. Murphy is more interested in investigating a Malibu address he finds than the dead body. However, two of Frank Vincenzo’s men arrive, and Murphy and Arabella leave by the fire escape. In front of the hotel, Murphy shoots the gas tank of his pursuers’ car. He and Arabella drive to Malibu to warn Albert Skinner, who was the prosecutor at Joan’s trial. As they search the many rooms of Skinner’s mansion, Joan takes Arabella hostage. In the study, Murphy finds Skinner suffocated with a plastic bag wrapped around his head. Upstairs, Joan has written a message in red lipstick on a mirror saying she is waiting for Murphy at the place where it all started. Murphy phones the station but instead of reaching Art Penney, he talks to his foe, Ed Reineke, and explains that Joan Freeman committed the murders. He asks for backup at the Bradbury building, where Joan killed her security guard boyfriend long ago and is now holding Arabella hostage. Reineke deliberately withholds the message, and tells Frank Vincenzo and his men to meet him at the Bradbury. Under instructions from Frank Vincenzo to capture Murphy, Reineke orders Murphy to drop his gun, but Joan kills him with a crossbow. She insists Murphy come forward, but when he reaches for his weapon on the floor, she shoots an arrow at him, but misses. When Murphy tries again, Joan’s arrow pushes the gun farther away. As Murphy hides, Frank Vincenzo and his men come looking for him. Inside, they find Reineke’s body In the dark, Vincenzo mistakes one of his own men for Murphy and accidentally kills him. Murphy ambushes a second thug, and steals his weapon. Frank Vincenzo sprays bullets into a body lying on its stomach. He turns it over and is surprised to also discover it is one of his men. Murphy appears around a corner and shoots Vincenzo dead. Above, Murphy watches as Joan places Arabella on the elevator and it descends. Murphy races down the stairs and rescues Arabella but as they crawl away Joan shoots Arabella in the back with an arrow, and retorts that the game is now between her and Murphy. She swipes at him with an axe, but he pushes her down the stairs. She ends up hanging off a balcony, gripping the axe. She pleads with him to save her, but Murphy just watches as Joan loses her grip and falls to her death. In an ambulance, Murphy and Arabella rest side by side on gurneys. Arabella is alive and spews vulgar language. Murphy threatens to wash every inch of her body with a bar of soap as soon as he is able. When she refuses to stop, he says forget the bar of soap, he will need a case of soap.