Plot
A week before LAPD Sergeant Roger Murtaugh's retirement, he and Sergeant Martin Riggs are demoted to uniform duties after failing to defuse an office building bomb. While on street patrol they witness the theft of an armored car, and help to thwart the crime assisted by armored car driver Delores. One of the two thieves, Smitty, gets away, but the other, Billy Phelps, is taken into police custody. The suspect is a known associate of Jack Travis, a former LAPD lieutenant who is running a local arms smuggling ring. The criminals were using armor-piercing bullets. Riggs and Murtaugh are re-promoted and assigned to work with Sergeant Lorna Cole from internal affairs to track down Travis.
Travis is negotiating with mobster Tyrone regarding his arms deal. Smitty is brought to Travis, who kills him for putting the police on his trail. Travis then uses his old police credentials to enter the interrogation room and kill Phelps in custody before he can be interviewed. However, closed-circuit cameras have since been installed in the station without Travis' knowledge, so Cole manages to confirm Travis' identity. While the three are reviewing the footage, their friend Leo Getz, who has been helping Murtaugh sell his house, arrives and recognizes Travis from prior business deals. Murtaugh, Riggs, and Getz narrowly miss capturing Travis at a hockey match, and Getz is wounded. Getz mentions a warehouse Travis owns, which is where he has stored his arms shipments.
Riggs and Murtaugh contact Cole for backup before they raid the warehouse, stopping at a food truck to wait for her. While waiting for their food, they witness a drug deal and attempt to cease it. Murtaugh kills a gunman who fired at them, while the others retreat. Afterwards, Murtaugh recognizes the gunman, his son Nick's close friend, Darryl. As Murtaugh is emotionally distraught, Riggs and Cole head to the warehouse and secure Travis' next arms shipment delivery. That night, Riggs and Cole have sex. He later finds a guilt-ridden Murtaugh drunk on his boat and consoles him in time for Darryl's funeral. There, Darryl's father insists that Murtaugh find the person responsible for giving Darryl the gun.
Cole discovers that Darryl's gun, the armor-piercing bullets, and the arms they recovered were originally in police custody, meant to be destroyed, and Travis stole them; they revoke his credentials from the system. They further tie the guns to Tyrone and interrogate him. He directs them to an auto garage from where many of Travis' henchmen work. Riggs, Murtaugh, and Cole manage to arrest several of the men.
Meanwhile, Travis has one of his men hack into the computer system to find another arms storage area. To steal the guns using Captain Murphy's credentials, he forces him at gunpoint to take him to this new facility. Cole finds the evidence of hacking and Murphy's absence, and the trio, along with rookie cop Edwards intercept Travis. They manage to rescue Murphy and engage in a shootout with Travis and his men before he can possess the weapons, but Edwards is killed by armor-piercing bullets during their pursuit. Travis also escapes.
Getz provides information on a housing development owned by Travis's shell company. Riggs, Murtaugh, and Cole infiltrate the site at night and find themselves in a gun battle. Riggs sets the construction site on fire and all of Travis' men are killed, while Travis wounds Cole. When Travis uses a bulldozer to chase down Riggs, using its blade as a bullet shield, Murtaugh tosses Darryl's gun, now loaded with the armor-piercing bullets, to Riggs, who shoots and kills Travis through the blade. After learning that Cole wore two layers of kevlar vests, Riggs admits his love for her as she is taken away in a chopper.
The next day, Murtaugh's family is celebrating his retirement, when Murtaugh reveals to his family and Getz that he has decided to not sell the house and stay with the force, preserving his partnership with Riggs.
