While Dutch and Claudette investigate the murder of Nancy Reborg, they discover that her daughter is missing and has been sold by her addict father to a child molester. David tries to crack corruption in the Barn by recruiting Terry to help him take down Vic.
It's warrant sweep day at the Barn. Vic and the Strike Team discover visiting NBA superstar Derrick Tripp hanging out with his "homies" before the evening game with the Lakers. Vic decides to do his part for the home team by keeping Tripp as a "friendly hostage". One of Vic's street informants, Connie, approaches him after having been roughed up by a new john. Vic hands off the case to Claudette and Dutch, who is convinced they're on the trail of a serial killer. Dutch's theories are proved wrong when he and Claudette ultimately discover that their perp is not a serial killer but rather a disturbed man who is raping women in a desperate attempt to procreate. Danny and Julien stumble upon a gun smuggling ring while delivering one of their warrants and disaster is averted only by Danny's sharp instincts. David has his hands full overseeing the sweep, managing an increasingly packed cage, soothing the frazzled nerves of the attending desk sergeant, and avoiding a potential lawsuit from Derrick Tripp's lawyer over the disappearance of his star client.
A dispute over an up-and-coming singer's success erupts in a gang war that threatens to bring Vic down, too; a case that could help Aceveda's political future sends Dutch and Claudette searching for a missing day laborer.
As Dutch's theory about a serial killer preying on prostitutes proves true, he convinces Aceveda to launch an all out effort to find him before he strikes again; Julien decides to bring down Vic for stealing drugs.
"The Shield" enters its fourth season with Vic Mackey adrift after his past actions have caught up with him. He's no longer the cop he used to be - the Strike Team has dissolved and he's stuck babysitting a menial surveillance sting that keeps him off the street. With Captain Aceveda moving on to his new City Council position, Captain Monica Rawling prepares to take over the Barn and implement her controversial anti-gang policies.
As Monica is forced to defend her crime-fighting tactics, Vic's pursuit of a killer creates problems for Antwon, while Dutch's secret deal with the D.A.'s office gets him and Claudette assigned to a murder investigation.
Shane's guilt for killing Lem intensifies after the Strike Team learns that he didn't make a deal with the authorities, and Kavanaugh resorts to threatening informants and planting evidence to pin Lem's murder on Vic.
Detectives discover the evidence Kavanaugh planted to make Vic look guilty for Lem's death, but Dutch begins to suspect that everything is too convenient. Meanwhile, Vic tries to find the Salvadoran leader Guardo, who escaped to Mexico.
Vic captures Guardo and tries to coerce a confession to Lem's murder out of him. However, Shane, the real killer, begs Vic for leniency on Guardo's behalf. Meanwhile, Dutch and Gardocki bring down a gang of meth dealers.
Claudette begins making radical changes to the Strike Team in anticipation of Vic's imminent forced retirement. Meanwhile, Vic's helplessness for not saving Lem boils over after he fails to play peacemaker in a gang war.
Shane is hospitalized after a beating, and his wife leaves him for having an affair. An unlikely suspect emerges in the serial rapist case, and an undercover agent in the Salvadoran gang tells Vic that Guardo didn't kill Lem.
After Vic learns that Guardo wasn't Lem's killer, he begins to suspect that Shane could have been responsible. Meanwhile, Vic does a favor for a city official, hoping it will give him an ally in his quest to avoid early retirement.
Shane contacts the Armenian mob and collects information against the Strike Team, in order to protect himself from Vic. Meanwhile, Vic learns that Hernan lied about the San Marcos massacre when the Byz Lats retaliate against the Salvadorans.
Shane offers his services to Kesakhian's daughter, who is running the Armenian mob for her sick father. Meanwhile, Hiatt tells Vic that Claudette has made sure that Vic's "retirement" from the Strike Team is a done deal.
Vic's city official can't help him out with his forced retirement, but he finds a new ally when he solves the San Marcos murders. Shane and Vic fight over Kesakhian's daughter's loyalties, and Billings gets revenge on Dutch.
Vic and Shane's relationship continues to deteriorate as they work against each other to track down the Armenian hit man. The war between the Mexicans and Salvadorans continues, and Vic gets a temporary reprieve from early retirement.
Vic's plan to play the Armenians against the Mexicans could backfire when the wrong man is targeted. Dutch plays hardball to get a witness to a homicide to step forward, and the mayor's new initiative to stop gang violence backfires.
Vic sets up a plan behind Shane's back to get the Armenians on their side. Meanwhile, Gardocki starts getting fed up about Vic continuing to work with Shane, and Dutch tries to get Danni a desk job to protect her.
Vic learns that the federal agent helping them out is in the "blackmail box" and has ties to Cruz Pezuela. Dutch tries to find out if he has a future serial killer on his hands, and Claudette's health is brought into question.
Vic launches his latest plan to get rid of the Armenian threat. However, the main difference in this plan is that, if successful, it will get rid of Shane too. Meanwhile for Shane, a ghost from his past reemerges.
Vic loses his hearing, which is brought up ahead of schedule, and faces early retirement. He also loses favor with Cruz Pezuela and begins playing hardball with Dani, who wants him to give up all rights to their child.
Shane's plan to get rid of Vic and Ronnie falls apart and threatens to be exposed. Cruz Pezuela asks Vic to strong-arm a priest, while David Aceveda joins Vic in working with the feds to bring Pezuela down.
Vic turns in his badge a week early and sets out to find and kill Shane and his wife. However, Ronnie is benched after Claudette shuts down the Strike Team. Meanwhile, Aceveda finds out more disturbing information about Cruz Pezuela.
As everything falls into place for the sting operation to bring Vic down, Shane is desperate for money. Back at the Barn, Ronnie tries to intercept a blackmail package, and Aceveda begins to question Pezuela's "suicide."
Mara breaks her collarbone and accidentally shoots and kills an innocent person when Shane's plan to get more money goes sideways. Meanwhile, Vic contemplates leaving Ronnie out to dry and taking a deal with the Feds without him.
As Vic hopes his immunity deal will help keep his family together, and Shane takes extreme measures to keep his pregnant wife from being charged with murder, Dutch finds he's a suspect in the disappearance of a teenage serial killer's mother.