In Los Angeles, California, Amanda Hunsaker throws herself off the balcony of a high-rise apartment in a drug-induced stupor. The next morning, Roger Murtaugh, a homicide detective for the Los Angeles Police Department (LAPD), is interrupted in the middle of a bath by his wife and children who bring him a cake for his fiftieth birthday. At breakfast, Roger’s wife, Trish, tells him that a man named Michael Hunsaker has been trying to reach him at the office. Roger says it’s been twelve years since he has spoken to Hunsaker, who fought with him in the Vietnam War. Later that day, Roger is called to the scene of Amanda’s suicide and meets Dixie, a prostitute who witnessed Amanda’s jump. When Roger learns Amanda’s name, he realizes she is the daughter of Michael Hunsaker. Meanwhile, Martin Riggs, a narcotics detective for the LAPD, reveals himself as a policeman during an undercover drug bust at a Christmas tree lot, and a gunfight ensues. After Martin shoots three of the four drug dealers, the other takes him hostage, and Martin dares the man to shoot before disarming him. That night, Martin contemplates suicide in his mobile home on the beach. After staring at a picture of his deceased wife, he loads a gun and points it into his mouth, but stops himself. The next day, Roger learns that Amanda’s case is now being investigated as a murder because the drugs she took were laced with toxic drain cleaner, and evidence shows that someone was in bed with her just before she died. Roger learns that he is getting a new partner, Martin, but when he sees Martin holding a gun outside his office, he mistakes him for a criminal and tackles him; however, Martin, an expert fighter, overpowers Roger within seconds. Later, as they leave the office, Martin explains that his superiors believe he is insane, or pretending to be insane in order to receive a pension, and Roger complains about having to work with him. At a meeting with Michael Hunsaker, Roger learns that Amanda was involved in pornography, and Hunsaker asks Roger to kill the people responsible for Amanda’s death. Roger refuses, but later tells Martin that Hunsaker saved his life in the Vietnam War. That afternoon, Martin and Roger are called to the scene of a suicide attempt, where a man named McCleary is standing on the ledge of a tall building. Martin joins McCleary and handcuffs him, warning him that jumping would drag them both down and thus be a murder and a suicide. When McCleary refuses to leave the ledge, Martin pushes him to jump and they both leap from the ledge at the same time; however, an air cushion has been set up below and they fall to safety. Roger confronts Martin about his risky behavior, and Martin reveals that he is suicidal, but that his love for police work has prevented him from killing himself. At the home of Amanda’s “sugar daddy,” a suspect in her murder, Martin saves Roger from being shot, but kills the suspect. That night, Martin joins Roger’s family for dinner, and tells Roger that he doubts the sugar daddy was the killer. As Martin leaves, he reveals that he was a sniper in the Vietnam War and believes that shooting was his only great skill. Roger later finds an evidence package addressed to him containing Amanda’s high school yearbook and a pornographic videotape she made with other women. The next morning, Martin wakes up Roger and proposes that Dixie, the prostitute who witnessed Amanda’s fall, was also her killer. When they arrive at Dixie’s house to investigate, however, the building explodes. Martin finds a trigger device amongst the wreckage, the same kind used by mercenaries during the Vietnam War. Neighborhood kids who witnessed the explosion tell the detectives that they saw a man at the gas meter outside Dixie’s house earlier that day, and he had a tattoo that matches one on Martin’s arm. Martin explains that the tattoo means a person was in the Special Forces. At Hunsaker’s home, Roger proposes that Hunsaker is involved in criminal activity that got his daughter killed, and that his daugher’s murderer was Dixie, a prostitute paid to poison her. Hunsaker admits to being part of a heroin smuggling operation with several other veterans who were part of Shadow Company, a special unit of mercenaries and assassins formed during the Vietnam War. A helicopter flies past the house, and Hunsaker is shot and killed by Joshua, one of the mercenaries from Hunsaker’s drug ring. At night, Martin seeks out Dixie on the streets; Joshua drives past and shoots, but Martin’s bulletproof vest protects him. Moments later, Roger gets word of a killing near his house and rushes home to find that his oldest daughter, Rianne, has been kidnapped. Mercenaries from the drug ring call, ordering him to go to Dry Lake in Victorville, California, at sunrise, saying they only want the information that Hunsaker gave him. Roger and Martin develop a plan to retrieve Rianne safely, knowing that Joshua believes Martin is dead. The next morning, Roger drops Martin off on the outskirts of the dry lakebed, then confronts the mercenaries, who arrive with a limousine, a truck, and a helicopter full of armed men. Roger threatens the mercenaries with a grenade, and the distraction allows Martin to begin sniping from across the lakebed. In the ensuing gunfight, Martin and Roger are captured by the mercenaries. In the back of a nightclub, the mercenaries torture Martin and Roger in separate rooms, wanting to find out whether Hunsaker revealed details of an upcoming heroin deal. Martin kills Endo, his torturer, then shoots his way through the building, saving both Roger and Rianne in the process. Joshua flees in a hijacked car and Martin gives chase on foot but loses him. Meanwhile, Roger shoots the driver of a car containing the mercenaries' leader, General McAllister, causing the car to run into a bus and explode. Back at Roger’s house, Joshua breaks in, but Roger and Martin intercept and disarm him. Before arresting Joshua, Martin challenges him to a fistfight and police officers surround the area. After an extensive physical battle with Martin, Joshua steals a police officer’s gun, but Martin and Roger shoot him before he can fire. On Christmas Eve, Martin delivers flowers to his dead wife's grave, and stops by Roger’s house to give his partner the bullet with which he almost shot himself, signifying that he is no longer suicidal. Roger insists that he stay for Christmas dinner and Martin accepts.