The fourth season of NCIS: Los Angeles premiered on September 25, 2012 on CBS and January 6, 2013 on Sky 1.
Following Callen's public shooting of the Chameleon, all is not well with the NCIS: LA team, as Callen is placed on suspension and Hetty adjusts to retired life.
A drone strike on a bomb-making compound in Afghanistan reveals the body of a recently retired Marine, leaving the NCIS: LA team to investigate why the Marine returned to Afghanistan and whether or not he betrayed the country he fought for.
The NCIS: LA team investigate a compromised intel operation after four people connected to the classified project are killed at a diner explosion. As Deeks and Kensi are on the trail of those involved, they learn that a wayward teenage girl might be related and Kensi can’t help but be empathetic to her situation.
The NCIS: LA team investigates the tragic death of a U.S. senatorial candidate’s staffer who is run over crossing the street. The high-profile case becomes a matter of national security as the agents must determine whether it is an isolated tragic incident or a politically connected plot that might put others in danger.
When a CIA agent who worked with Sam Hanna over eight years ago seeks his help and is soon found dead of an apparent suicide, Hanna and Callen suspect foul play and a bigger conspiracy. Deeks finds his inner 'dog whisperer' with Kensi as the two go undercover and enter a dog show with a prized poodle.
The team continues their investigation into the death of a CIA agent, that leads to a dangerous ring of Russian super agents. In a race against time, Hanna risks having a secret exposed.
The NCIS: LA team heads to the beach to question the possible link between a surfer, an Internet video and a dead Naval tech scientist. Meanwhile, as Callen and Hanna investigate the victim’s intelligence connections, Deeks and Kensi go undercover to catch their suspects and some waves.
The NCIS: LA team questions the motive behind the murder of a retired CIA agent turned millionaire and in the process also questions what Hetty knows about the case.
The NCIS: Los Angeles team investigates the strange and brazen heist executed by gunmen in costumes that could directly impact the U.S. economy. Meanwhile, as Deeks testifies on a case, Granger insists on partnering up with a very reluctant Kensi.
Callen, Sam, Kensi and Deeks spend Christmas aboard an aircraft carrier investigating the death of a fellow NCIS special agent. Meanwhile, Nell and Eric make the most of the holidays back at headquarters and Hetty takes a trip.
A former defendant of Deeks is targeted for discovering information that could dismantle a lucrative car shipping operation with links to a global terrorist ring. Meanwhile, the NCIS: LA team tackles the large-scale stolen car operation from the inside when Kensi goes undercover in an auto body shop.
A distraught widow is suspicious of the circumstances surrounding her Marine husband’s last battle and Sam and Callen help to determine if there has been a cover-up. Meanwhile, Hetty summons Operational Psychologist Nate Getz to be her “pawn” and the rest of the NCIS: LA sense another evaluation on the horizon.
Callen goes undercover to infiltrate a network of Chechen terrorists who are recruiting foreign fighters to join their cause and planning their next attack in the U.S.
Callen, Sam, Kensi and Deeks go undercover as an elite tactical force in order to investigate an ambush tied to a cartel kingpin with terrorist connections.
An NCIS: LA investigation leads the team to question a lone survivor of a former terrorist organization, while Kensi and Deeks follow the trail into the woods
Callen is concerned that Sam might be too invested after he takes on a missing persons case for an Afghani elder who helped him years ago when he was wounded on a mission.
Sam fears for his family’s safety when his wife is called to resume a former CIA cover after a case suggests ties to the infamous Sidorov and his stolen nuke.
Callen and Sam fly to Idaho and join forces with NCIS: Red, a mobile team of agents who live together and investigate crimes in various locations, when a murder weapon found there matches one used in a national security case in Los Angeles.
While the LA team chases leads digitally and locally, the Red team travels in their mobile trucks, nicknamed 'Laverne and Shirley', in a race to track down a cunning terrorist before he disappears.
When cyanide in a communal water jug lethally poisons a lieutenant and leaves a Navy pilot in serious condition, the NCIS: LA team investigates whether this is an isolated incident or an early test for large-scale water contamination.
A body is stolen after a special task force successfully takes down an infamous cartel boss, leaving the NCIS: LA team to investigate the leak and track down the missing body.
Callen questions Hetty’s motivations when a bizarre case involving a mysterious woman is revealed to be an undercover agent from her past.
Deeks goes undercover to obtain information about an infamous arms dealer looking to get back in the business, but the woman he befriends leads the operation in a dangerous directon. Meanwhile, Kensi confronts new emotions as she runs surveillance on Deeks and his female source.
A nuclear explosion in the Pacific reignites the search for stolen nuclear weapons and prompts Hetty to shuffle the NCIS: LA partnerships, sending Callen and Kensi overseas and leaving Sam and Deeks teamed up state side.