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.

Against Murtaugh's advice, Cole reaches out to Tom Barnes to help with a case that hits close to home, putting Murtaugh and Cole's partnership to the test. Meanwhile, Trish prepares to announce her campaign for District Attorney.

When Cole and Murtaugh investigate a case involving a father and daughter crossing the Mexican border, they end up uncovering a major issue with California Boarder Patrol. Meanwhile, Trish prepares her campaign for D.A. and must face parts of her past, before they find their way into the media spotlight. Cole also confronts his past when he experiences flashbacks as a result of an injury.

Murtaugh and Cole investigate the murder of an underground MMA fighter when Murtaugh realizes his fighting moves aren't as sharp as they once were. Meanwhile, Trish is unfulfilled by her practice and considers running for a public office. Also, Cole and Erica try to keep things casual and uncomplicated but their plan backfires when Maya reacts to her family changes in a negative way.

Murtaugh takes a family vacation, but is pulled back into work when he witnesses an alleged abduction. Cole is left to investigate a murder himself, but the two cases begin to intersect. Meanwhile, Erica's mother, Senator Malick, comes to town and stirs things up between Erica, Cole and the case at hand.

Cole finds himself on the rebound after Natalie's engagement, and his new romantic interest, Julie, has an unexpected connection to A.D.A. Erica, complicating things at work. Meanwhile, Murtaugh worries about the connection between the murder he's investigating and a new opportunity for Trish to serve as co-counsel on a high-profile lawsuit with Leo Getz.

While Cole gets into the holiday spirit for his first Christmas with Maya, Murtaugh hopes he can convince RJ to come back home from Costa Rica for the holidays. Cole and Murtaugh are saddled with a complicated burglary and home invasion case. Avery's holidays are a bit blue. Bailey is still trying to figure out her new partner.

Cole wants to talk to Natalie about their relationship before it's too late, but a wrench is thrown in his plans after he bumps into a new ADA, Erica. After being kicked out of the house for Trish's girl's night, Murtaugh finds himself an unlikely friend.