Vic sets out to revenge Lem's death, even though Kavanaugh knows exactly what he's up to; Shane's increasingly bizarre and violent behavior has Vic and Ronnie concerned; Dutch and Billings find the San Marcos mass murder; Julien and Tina's relationship takes a turn for the worse during a robbery investigation at a porn shop.
Vic kidnaps Guardo's girlfriend in order to get a face-to-face meeting with the crime boss. Meanwhile, the San Marcos murder is dumped on the Barn's lap, and the sudden percentage increase in homicide numbers for the precinct puts Claudette under a political microscope; Danny is offered a promotion to sergeant.
Vic traps Guardo and tortures him over Lem's death. Meanwhile, Ronnie takes the lead in an undercover case involving meth dealers; Dutch misreads Tina's words and thinks she wants him; Claudette reveals to Vic that he's going to step down in one month's time, and that he needs to begin training his replacement; and, when Guardo lies during his "interrogation" Vic puts a bullet in his brain.
Claudette wants Vic to train his eventual replacement or risk losing his job immediately. Meanwhile, Dutch and Claudette are confronted with an unsolved case from their past wherein a rapist carves the words "Go Home" into the stomachs of his victims, all young runaways; and, Acevada looks to further his political career.
New Barn member Kevin tracks down an undercover federal agent who sheds light on the San Marcos murders: it seems the Mexicans and the Salvadorians got into a major battle.
Vic and the team track down the man who killed the daughter of a city councilman. They need to tread lightly, though, as learning about the daughter's proclivities would shatter their perfect image of her and put the entire Barn at risk.
Shane compiles a report on all of Vic's sinister deeds over the years and gets in good with the head of the local Armenian mafia all in an effort to cover himself in the event Vic comes after him or his family.
Shane tries to head off the Armenian mob's reaction to his murder of an Iraqi gas station owner by meeting with their leader. Instead, he gets the mob boss's daughter, who may not be as in the dark about mob dealings as she initially seemed to be.
Vic and Ronnie get a tip concerning one of the thugs involved in the San Marcos murders and track him down. Vic makes a deal with the guy to leave him be once he can finger the others responsible.
Vic and Aceveda form an uneasy alliance to stop a lynchpin for an international mob. Vic also wants Aceveda's help in getting through his performance hearing, which could be the end of his career.