Officers in the Barn investigate a woman's disappearance and the arrival of a large drug shipment; lives change forever.

As Vic prepares to help Lem get out of the country to begin a new life in Mexico; Kavanaugh and Aceveda attempt to capitalize on the situation by using this as means to try to take down the leader of the Strike Team. Dutch decides to file for a transfer. Meanwhile, Shane tries to cut off the investigation by taking extreme measures and Danny finally confronts the father of her child.

Olivia joins forces with the Strike Team to catch a Mexican drug king and his shipment. Meanwhile, Dutch cleans up Billings' mess, and the death of a woman associated with the Strike Team prompts an investigation by Claudette.

Vic redirects his attention to finding out who really killed Lem after he learns for certain that Guardo wasn't involved. The Strike Team investigates the murder of a politician's daughter. Later, Cassidy confronts her father about his unethical past.

A local priest may be protecting drug dealers; Dutch and Billings close in on attempted cop killers; Corrine learns uncomfortable truths about Vic.

When Aceveda confirms that the Strike Team is the target of an Armenian mob hit, Mackey must take matters into his own hands. Meanwhile, Whym's career is on the line when she disobeys Aceveda's orders to put a case to rest that may prove the innocence of convicted felons.

With the Barn grieving over the loss of one of their own, Vic's out to avenge his friend's murder, but finds himself in the crosshairs of Kavanaugh. Claudette assigns the Strike Team to investigate the torching of a methadone clinic. Meanwhile, Julien and Tina look into a robbery/assault at a porno shop, and Dutch makes a horrifying discovery in San Marcos.

Kavanaugh acquires what he needs in order to apprehend Vic who is preoccupied with getting revenge on Guardo. Claudette finds herself under the political microscope as a result of the San Marcos incident, and Danny receives some news about her future at the Barn.

After Vic disowns him, Shane turns to the Armenian mob for help. Vic gets more involved with the San Marcos investigation, while Dutch and Billings are able to track down someone who survived the massacre.

Claudette notices Hiatt's mistake when he makes an arrest, and Billings delivers shocking news to the department. Vic forms an alliance with Aceveda, while he waits for his review board hearing. Meanwhile, Shane's life keeps spiraling out of control.

In the aftermath of a riot, cops are being lured into ambush shootings by a rash of bogus 911 calls. Vic and Aceveda are forced to work together to stop the attacks and solve the problems in the Grove, while Gilroy attempts to pit the two against each other in an effort to save his own job.

Kavanaugh seeks the help of Antwon Mitchell in his ongoing quest to take down the Strike Team. In the midst of this, Kavanaugh has a lot on his mind as his ex-wife reenters the picture, and he reluctantly agrees to let Vic send Emolia undercover to infiltrate a Salvadoran gang that is stockpiling grenades.

Claudette gives Vic a chance to save his job if he agrees to train his potential replacement. An unsolved rape case from the past that Dutch and Claudette were working on resurfaces. Meanwhile, Aceveda meets someone that can help him further his political career.

As progress is being made in the San Marcos investigation, Vic receives bad news about his career. Meanwhile, Shane and Diro become business partners as she prepares to take over the Armenian mob.

As Vic gets closer to finding his family, the Strike Team focus their attention to Armadillo after he burns down Tio's business, an act that threatens to expose Vic's protection racket. As Claudette gets closer to the truth about Vic's connection to Tio, Dutch is accompanied by the civilian auditor to solve the murder of a meter maid, while Danny and Julien intervene between two feuding neighbors.

When a gang execution hits close to home for Claudette, Vic, Tavon, and Dutch work together to find the culprit before anyone else gets hurt. Shane, Lem, and Ronnie are forced to choose between hitting the money train without Vic or letting it go. As Aceveda prepares for the city council election, Danny receives some life-changing news, and the harassment inflicted upon Julien reaches a whole new level.

With an Armenian hit-man hot on their trail, the Strike team sweeps the streets of the Armenian community to find him before he finds them. After catching a break, Aceveda's suspicions grow about the Strike Team's involvement with the Money Train robbery. Meanwhile, Dutch and Claudette investigate a case that could repeal hundreds of convictions, which leaves Claudette having to choose between her conscience and her career.

Via a bug planted in the clubhouse, Kavanaugh monitors the Strike Team as they investigate an illegal prescription drug ring that is proving to be deadly for the people of Farmington. While working on a missing person’s case, Dutch attempts find out what Claudette is hiding.

After being badly beaten, Shane's life spirals even more out of control as his guilt continues to eat away at him and his marriage begins to fall apart. Vic and Ronnie try to learn who was responsible for Shane's attack. Meanwhile, under the new leadership of Kevin Hiatt, Claudette assigns the Strike Team to investigate the San Marcos murders. Dutch and Billings make progress in a serial rapist case.

As Dutch uncovers a lead to the serial killer, a reporter's questions force Aceveda to explain problems he thought were buried fair in the past, while Vic assists Connie as she tries to quit crack cold-turkey in a desperate attempt to keep her son, leaving the other Strike Team members to investigate a strip-club mugging ring on their own.

When Vic discovers that Ronnie has been burned as payback, the rest of the team set their sights on taking Armadillo off the streets for good. Meanwhile, Claudette makes headway on her investigation into Vic after a conversation with Corrine, and Dutch takes Danny under his wing as they attempt to track down the suspect of a Thai Town beating.

To the dismay of his friends, a weary Lem is thinking about accepting a deal from the D.A. where he has to plead guilty to the heroin charges. With no other choice, Vic reluctantly meets with Antwon in order to ensure Lem’s safety in prison. Meanwhile, unforeseen circumstances that are beyond Claudette and Dutch’s control might cause irrevocable damage to their friendship.

Consumed by bloodlust, Vic finally gets the chance to avenge his friend's murder, despite Shane's pleas against it. Meanwhile, Ronnie is assigned by Claudette to help Dutch and Billings investigate a robbery homicide at a pharmacy.

Vic sets into motion plans to end the gang threat as his relationship with Cassidy deteriorates; Dutch and Billings investigate a possible sleepwalking murderer.

Politically ambitious new LAPD captain David Aceveda tries to crack corruption in Los Angeles' Farmington District by recruiting detective Terry Crowley to help him take down fellow detective Vic Mackey, leader of the Farmington precinct's Strike Team. As Vic and the Strike Team prepare for a dangerous night mission, detectives Holland "Dutch" Wagenbach and Claudette Wyms investigate a woman's murder, only to discover that her missing daughter has been sold off by the daughter's drug-addicted father to a child molester.