Callen and Sam are trapped in an armed submarine that terrorists have aimed to hit an aircraft carrier in San Diego. Meanwhile, Hetty defies orders and forgoes her trip to Washington, as the team tries to locate and save Callen and Sam and prevent the terrorist attack.
The NCIS: LA team discovers that one of their own is compromised when they investigate the murder of a Navy contractor involved in illegal transactions of confidential data. Meanwhile, Hetty is under investigation in Washington, D.C., when the Department of Justice arrives in L.A. to investigate the team.
The NCIS: LA team takes action to protect one of their own while being interrogated by the Department of Justice. Meanwhile, Nell goes undercover as Hetty, and Director Leon Vance gives Hetty disturbing news while she is in Washington.
The team goes beyond boundaries to make sure Hetty is safe but what they don't know is that Hetty have her own plan as always. Meanwhile Nell is having a hard time after having killed for the first time and is visited by a know face who might be able to help her.
When Sam is arrested for murdering a woman, Callen and Granger work to clear his name. Meanwhile, Kensi and Deeks discover a link between the victim and a student from Sam's past. Anderson Silva guest stars.
When a man is murdered by a small drone, the team discovers his connection to a terrorist group that is planning another local attack. With time running short, Nell must use her drone expertise to try to divert a disaster in Long Beach.
The team investigates a burglary at a cyber-security company, but Callen must steer clear of the crime scene after Hetty reveals his girlfriend, Joelle, is one of the lead witnesses. Also, the team discusses their personal holiday travel plans. Kensi and Deeks are given the chance to get even closer to each other.
Callen and Sam are kidnapped when they follow an anonymous tip about a missing NSA agent. When the team discovers they are nowhere to be found, they realize it could have been a trap set by terrorists.
On what is supposed to be a routine op, Sam takes a bullet and the team sets out to find who the shooter was; Kensi and Deeks have their first argument and Kensi starts to think they may be moving too fast.
Callen and Sam continue their undercover mission to take down a white supremacist group that it's believed to have procured nuclear material, while Kensi poses as Callen's tattooed biker girlfriend; the moment Callen and Hetty first met is revealed.
Callen and Sam are surprised when a murder victim turns up alive and wants their help. The NCIS OSP team investigates and finds that someone has broken the rules of sanctions against Russia. Arkady's in trouble again.
An ongoing case leads Callen, Sam, Deeks and Kensi to Moscow where the team must assume multiple undercover identities to locate their suspects. Also, Callen uncovers new information regarding his father.