A slightly unhinged former Navy SEAL lands a job as a police officer in Los Angeles where he's partnered with a veteran detective trying to keep maintain a low stress level in his life.

A noise complaint at a boxer's home leads Riggs and Murtaugh to a case involving illegal gun runners and Riggs becomes personally invested in the outcome when one of their targets reminds him of his late wife. Trish worries about Murtaugh's health.

Riggs makes a shocking discovery about Miranda’s death that leads him back to the cartel in a frantic search for answers. His wild desperation leaves Murtaugh torn between his commitment to his partner and his commitment to his family.

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. Avery is questioned by Internal Affairs about the integrity of his department.

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. Trish gets caught up in some trouble. Molly asks Riggs to make a big decision about their future.

Former international CIA operative, Wesley Cole returns home and assumes duty as a police officer with the LAPD. Cole will find himself navigating a new partnership with Murtaugh, as he adjusts to life in Los Angeles. Avery continues his campaign for city council.

When Cole is investigated by the FBI, Murtaugh is forced to question his loyalty to his own partner. Meanwhile, Cole works a case that puts Natalie's fiancé, Andrew, into the crosshairs right before their wedding day.