Plot

The story begins on the night of December 14, 1987 with the apparent suicide of Amanda Hunsaker (Jackie Swanson), who jumped from an apartment balcony after a cocaine session. Amanda is the daughter of Michael Hunsaker (Tom Atkins), an old friend and former army buddy of LAPD Detective Sergeant Roger Murtaugh (Danny Glover). Murtaugh has just turned 50 years old. Murtaugh finds that Amanda did not commit suicide. Her pills were full of drain cleaners and would have killed her anyways. Murtaugh talks to Hunsaker who says that Amanda was into making pornographic films.

A veteran cop of 20 years, Roger Murtaugh gets stuck with a new partner on his 50th birthday. That partner is 37-year-old Detective-Sergeant Martin Riggs (Mel Gibson), whose wife was killed in a car accident three years earlier. As a result, Riggs is a suicidal, borderline alcoholic, who has become so reckless and violent that he is considered a "lethal weapon". The Psychologist (Mary Ellen Trainor) at the department believes that it is a mistake to keep Riggs in the field. But Captain Ed Murphy (Steve Kahan) refuses to take Riggs off the front lines.

Sergeant Murtaugh first believes that his new partner is "trying to draw a psycho pension" and is horrified to learn that he genuinely is "crazy." Riggs lives in a trailer park and his accommodations are downright filthy. He takes unnecessary risks at work, and it is almost as if he wants to get killed in the line of duty. Riggs volunteers to talk to a suicide jumper on a ledge and instead of saving him, Riggs handcuffs him and challenges him to jump together. Although, Riggs claims that he was controlling the jump to the air mattress below, but Murtaugh is livid that Riggs is trying to kill himself.

However, Murtaugh is soon indebted to his new partner for saving his life. Murtaugh and Riggs go to a club which Amanda visited often and are attacked by an unknown man. Murtaugh shoots the person in the leg and wants to question him, but the man pulls a gun on Murtaugh and only quick action by Riggs saves Murtaugh's life. Murtaugh introduces Riggs to his family, including his wife Trish (Darlene Love), daughter Rianne, son Nick (Damon Hines), daughter Carrie (Ebonie Smith).

Riggs reads Amanda's report which says that someone spent the night on her bed with her, on the night she died. And a prostitute named Dixie (Lycia Naff) saw the whole thing. Riggs reckons that maybe Dixie was the one in Amanda's room, who poisoned her pills and saw her jump. Dixie had to rush out of the hotel room, and people saw her and so she reported the incident to the police to cover her own behind. Before they can reach Dixie, her house explodes in a ball of fire. A nearby kid witnesses a man go by her house earlier in the day, who had a Special Forces Tattoo.

While investigating Amanda Hunsacker's death, the two uncover a heroin-smuggling operation run by Vietnam War special forces troops, known as 'Shadow Company' (see Air America). The scheme is masterminded by a ruthless retired General, Peter McAllister (Mitchell Ryan) - the former commander of Shadow Company - and his chief enforcer, Mr. Joshua (Gary Busey).

Murtaugh and Riggs - also Vietnam War veterans - recognize the tattoo. Murtaugh confronts Michael and discovers that Michael Hunsacker was laundering the profits through his bank. McAllister had ordered the murder of his daughter to keep his laundryman under control. Michael says that he has another daughter to protect. Mr. Joshua makes a surprise appearance in a helicopter and shoots Michael in the back with a sniper rifle. Joshua reports to Peter that Michael spoke to the police before being killed.

As Riggs and Murtaugh discover more about Shadow Company's network, the increasingly violent members of Shadow Company make an attempt to kill Riggs (Riggs survives only because he was wearing his vest), then kidnapping Murtaugh's daughter (Rianne (Traci Wolfe)) after Riggs and Murtaugh discover her boyfriend was killed by Shadow Company. Despite their best attempt to free Rianne by using Riggs presumed dead status to their advantage, the three are captured. Using Rianne, Joshua pressures Murtaugh to reveal which of their drug shipments have been compromised. Riggs, though initially captured by McAllister and brutally tortured with electric shocks, frees himself and then liberates both Murtaugh and Rianne. The pair then declare war on Shadow Company, gunning down most of its members.

McAllister is caught in a car accident by Murtaugh and is blown up by a live hand grenade (along with his heroin shipment). Both Riggs and Murtaugh shoot Mr. Joshua after Riggs beats him with a Triangle Choke in an unarmed combat fight on Murtaugh's front lawn.

Murtaugh and Riggs are now solid friends, and Riggs spends Christmas Day at the Murtaugh home with Roger's family; Riggs brings his dog Sam to be a friend to the Murtaugh family cat, Burbank. Riggs also gives Murtaugh a symbolic gift, the unused hollow point bullet which he had been saving to commit suicide.