Flashback to El Paso, Texas where Riggs and his partner are chasing down a group of drug runners when Riggs' wife calls to tell him she's going to the hospital to have their baby. Riggs pulls over and shoots at the runners causing their truck to flip over. Miranda is headed to the hospital when he is t-boned by a truck. Riggs leaves and heads to the hospital where he finds out his wife and unborn child have died.
Six months later, Murtagh is getting ready to go back to work after having a near-fatal heart attack while in the delivery room as his wife, Trish, was giving birth to their youngest daughter. When he gets to work, Captain Avery calls him into his office to tell him he has a new partner.
Murtagh is called to a robbery. Riggs is already there, but he's going into the bank while Murtaugh is trying to figure out what he's doing. The other cops tell him that's his new partner. Riggs walks into the bank with pizzas and the robbers make him kneel on the ground. Riggs surveys the situation, tries to negotiate, then takes them down. One of the robbers had a bomb and sets off the timer as he goes down. Riggs gets everyone out and casually strolls out introducing himself to Murtaugh when the bomb goes off.
Riggs is in Dr. Cahill's office discussing the incident while Murtaugh is in the captain's office complaining about Riggs.
The two are then called to their next case, an apparent suicide. When they arrive they get all the details from the department's pathologist, Scorsese, including the fact the gunshot was done with the right hand. Murtaugh starts walking away assuming it's wrapped up, but Riggs grabs the Ramone's (the dead guy) dog tags and tells Murtaugh they need to do a proper investigation. They visit Ramone's wife who tells them they were about to move to Arizona. Before leaving, Murtaugh learns that Ramone was left-handed. The two visit Ramone's job where his boss claims he didn't know him very well.
Riggs sees a guy with a familiar tattoo and goes to question him, but the guy takes off running. Riggs and Murtagh jump in their car and take off after him. The chase leads them to the Grand Prix. Riggs jumps out onto the bad guy's car and knocks him out causing the car to crash outside the track. When the guy takes off running, he gets hit by a bus.
Later, Avery chastises the two of them for the mess they created and tells Murtaugh he's been reassigned. Murtaugh and Riggs have a tiny tiff. At home, Murtaugh is cooking dinner when Riggs shows up with some new information. Trish invites him in and Riggs joins the Murtaugh family for dinner.
After dinner, the duo head to the morgue to get some more information from Scorsese. Riggs heads to Ramone's house where he finds out Ramone's son has been kidnapped because the drug runners think Ramone stole some heroin. Riggs heads back to his place to get some guns when Murtaugh shows up with a file showing the boss who claims he didn't know Ramone very well served with him in Iraq. Riggs doesn't want Murtaugh going to the port and handcuffs him to the counter.
At the port Riggs gets into a fight with some thugs who are about to kill him when Murtaugh drives his truck through the warehouse window saving Riggs. They grab the kid and take off, but a sniper shoots Riggs. Murtaugh only has one bullet in his gun and Riggs suggest sacrificing himself to save the boy. Murtaugh ends up shooting Riggs in the foot and Riggs grabs the other gun on the pavement and kills the sniper.
Later, Riggs brings some booze to Murtaugh's house and Trish invites him in again.