Murtaugh follows Riggs down to Mexico, where he plans to avenge his wife's death and finish off Tito Flores once and for all. Once back in the States, they have a few people to answer to during the ongoing investigation around Flores, including Murtaugh's family. Meanwhile, Avery is questioned by Internal Affairs about the integrity of his department.
Murtaugh follows Riggs to Mexico to pursue the dangerous drug cartel who Riggs believes is responsible for the death of his wife. The two have departed without the blessing of Captain Avery Brooks, and have left behind their LAPD colleagues Dr. Cahill, Scorsese, and Detective Bailey.
Murtaugh and Riggs investigate the mysterious death of famous singer Shaye's bodyguard. Murtaugh turns to his daughter, a fan of the singer, to learn more about Shaye's background and uncover the possible suspects. Meanwhile, Riggs learns the responsibilities that come with being in a relationship with help from Murtaugh.
Murtaugh and Riggs respond to a robbery that took place 20,000 feet in the air. The investigation turns into an ongoing hunt for an unreliable father, with his daughter being their main witness. Meanwhile, Murtaugh faces hostility from his own daughter, Riana, as he tries to connect with her.
Murtaugh and Riggs investigate a death that occurs moments before the victim's bet wins a horse race. Leo Getz insists it's a homicide and aids in the investigation. Meanwhile, Murtaugh updates his will, while Riggs struggles to come to terms with his past.
Riggs is reunited with childhood friends and comes to realize they might be mixed up in the robbery-homicide case he and Murtaugh are investigating. Meanwhile, Murtaugh finds a joint stashed in his kitchen that he brings to Scorsese for testing in effort to bust one of his kids.
Riggs and Murtaugh investigate the death of a surfer. Meanwhile, Murtaugh grows suspicious of Trish's new friend, with whom she's been working on a charity event - and then learns it's NBA champion Scottie Pippen. Also, Riggs has violent nightmares that bring up scars from his childhood.
Riggs and Murtaugh are led into the underground mob world as they investigate a murder in a hospital. Meanwhile, Murtaugh learns that Riana is secretly dating his neighbor and nemesis' son. Also, Riggs spends more time with his childhood friend, Molly, and Scorsese's real name is revealed
Riggs and Murtaugh are drawn into a fatal world of conspiracy theories. Meanwhile, Riggs' relationship with Molly evolves and Murtaugh and Trish attempt to get Harper into a fancy pre-school.
It's Christmas time and the Murtaugh family is headed to Hawaii. However, Murtaugh's vacation is put on hold when he and Riggs are called to a homicide and the victim is an old friend. Meanwhile, Riggs gets a surprise phone call from a relative just in time for Christmas.
Riggs and Murtaugh are thrown into the world of counterfeit money. When a group of teenagers inadvertently gets involved, it strikes a chord with Murtaugh, who is dealing with his own issues regarding his son, RJ. Meanwhile, Riggs decides to pay a visit to someone from his past.
When Riggs visits his father-in-law in prison, an old secret about his deceased wife comes to light; Murtaugh tries to impress Capt. Avery; Trish and Leo Getz try to solve a murder on their own.
Murtaugh and Riggs respond to the murder of an escaped patient of Dr. Cahill's who mysteriously goes off the rails. The investigation brings to light a hidden scheme far bigger than they imagined. Meanwhile, Riggs' anger gets the better of him once again, causing Avery to consider whether or not he is fit to serve. Also, family tension runs high between Murtaugh and Trish, who have to make big decisions about R.J.
Detective Bailey's sister runs into trouble with the law, and she must decide how far she'll go to protect her family; Murtaugh must help Riana navigate boy problems; Riggs deals with the fallout of a night of heavy drinking.
Murtaugh and Riggs search for the culprit of a local jewelry heist. Meanwhile, Murtaugh gets called “grandpa” by another parent while at the playground with Harper, sending him into a tizzy, and Riggs deals with withdrawal from drinking.
After the city's gun buy-back program is robbed, the team is on a hunt for those responsible. Meanwhile, Murtaugh walks on thin ice with Trish, Riggs finds an unlikely friend in Ruthie, and Captain Avery is reunited with a college “frenemy” who has key information about the case.
After the fatal end to an argument on a construction site, the squad works to untangle facts about the incident. Meanwhile, Riggs struggles to rekindle his relationship with Molly, Trish's job is on the line as she tries to close a big deal at work, and Murtaugh gets one last chance to make it right at home.
The duo sets out to unlock a case about a veteran hitman who Murtaugh has followed for years. Meanwhile, Riggs struggles to find a connection with his father, Murtaugh scares Riana with his overprotective driving lessons, and the team finds themselves wedged in a dangerous incident that tests their trust.
When Riggs and Murtaugh attend Leo Getz's surprise wedding, they end up investigating a missing person. Then, Riggs comes face to face with someone from his past and Murtaugh receives news that might change his life forever.
After a mansion break-in turns into a homicide, Murtaugh and Riggs set out to find the culprit. Murtaugh and Trish question their trust in Riana as she gets into some trouble with new friends. Meanwhile, Riggs recalls his troubling childhood and finds a true family with Molly and Ben.
On Murtaugh's first day on the job as interim captain, the wife of a popular businessman is abducted and the squad finds a connection in the case to Rigg's father. Meanwhile, Riggs is left to grapple with his family and work.
The squad is on high alert after an apparent threat is made on Riggs' life. With Murtaugh taking on more responsibility at work, he and Riggs have to deal with their changing relationship. Meanwhile, Trish gets caught up in some trouble and Molly asks Riggs to make a big decision about their future. Season Finale.