Politically ambitious new LAPD captain David Aceveda tries to crack corruption in the Barn by recruiting Terry Crowley to help him take down fellow detective Vic Mackey, leader of a police Strike Team consisting of himself, Shane Vendrell, Curtis "Lem" Lemansky, and Ronnie Gardocki. Meanwhile, detectives Dutch Wagenbach and Claudette Wyms investigate a woman's murder only to discover her missing daughter has been sold off by her addict father to a child molester.

The Internal Affairs Division begins an investigation into the shooting and death of Terry, while Aceveda starts his own. Meanwhile, Dutch and Claudette investigate the shooting of a churro vendor that ends in the Strike Team busting up a gang initiation.

Vic's relationship with Shane becomes even more complicated when the pair hook up with a cop from another precinct to investigate a shooting. Meanwhile, Dutch and Claudette bring in a psychic on a complaint from the son of one of her clients.

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 tries to pit the two against each other in an attempt to save his own job.

While Vic searches for his wife and children, the Strike Team allow more drugs into Farmington through a corrupt connection. But when their latest shipment turns bad, the Team heads to Tijuana to uncover the person behind the poisoned cocaine. Meanwhile, Dutch and Claudette try to track down the man responsible for fire-necklacing the leaders of two separate gangs.

Detectives Wagenbach, Mackey and Wyms investigate a massacre at a women's shelter, and Captain Aceveda works with Lowe on the case of a stolen bicycle.

Flashback to the first day of the barn. Under pressure to get results, Vic and his new Strike Team make an impression on the streets. Dutch and Claudette realize they make a good team, and David discovers he doesn't have as much power as he thought he would.

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, Lemonhead, and Ronnie are forced to choose between hitting the money train without Vic or letting it go. Meanwhile, Danny receives some life changing news and Julien's harassment reaches a whole new level.

The Strike team's attempt to play by the book comes to an abrupt end when they discover that two gangs, the Byz Latz and the One-Niners are attempting to alter the balance of power by acquring MP5s. Claudette and Dutch investigate the murders and feet decapitation of 4 Armenian men by the Armenian mob in their search for the stolen cash from the money train. Meanwhile, Danny who is doing temp work at a high school while waiting for an appeal, discovers she might me be able to get her old job back.

Vic struggles with Claudette's new supervision of the Strike Team. Marked bills lead Aceveda and a federal agent closer to the truth about the Armenian money-train heist in an episode with a recurring motif of people not reporting crimes against them.

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.

Monica orders her units to turn the streets of Farmington inside-out when two officers mysteriously disappear after answering a 911 call. Shane tries to make amends with his old crew after Vic agrees to help him get out from under Antwon, but things don't work out as planned. Dutch and Claudette convince Monica to let them take active roles in the case of the missing officers, but soon find themselves at a dead end with the only witness being clinically insane.

Monica, Vic and the strike team work together to put Antwon Mitchell back behind bars after his deal with the DEA released him from prison. When Vic reveals his suspicions of David's connection to both Antwon and an inmate's murder, David must protect his political future at any cost. When the murder of an over-protective father leads Claudette and Dutch back to a foster home they had previously investigated, Monica makes an unpopular decision that helps seal her fate.

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.

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.

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.

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.

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