As the sixth season opens, Vic is obsessed with finding Lem's killer, and Kavanaugh is obsessed with pinning the murder on Vic. Claudette wants Vic off the case, so she assigns the team to a fire at a methadone clinic. She also tells him the brass wants him to retire. Meanwhile, Shane is behaving erratically; Corinne confronts Danny; Julien and Tina respond to a robbery at a porn shop; and Dutch comes across a crime scene so gruesome that even veteran cops have trouble stomaching it.

A warrant for Vic's arrest for Lem's murder is issued as a result of Emolia's statement and evidence Kavanaugh uncovers in Vic's house. But that's not stopping Vic from his single-minded determination to get revenge on Guardo. "Every second he breathes and Lem doesn't is unacceptable," Vic tells Shane and Ronnie. Meanwhile, the Barn is getting the blame for the San Marcos murders, and Danny receives some good news---or maybe not---when she visits the Barn with her baby.

Shane can protest only so much when Vic catches up with Guardo and turns up the heat on the man he thinks is Lem's killer. Ronnie, meanwhile, is assigned to work with Dutch and Billings on a meth-murder case. Tina is also involved, and Dutch mistakes her interest in his work for something else. And Claudette tells Vic about an upcoming addition to the Strike Team.

Vic's replacement, Det. Kevin Hiatt (Alex O'Loughlin), arrives at the Barn, and Claudette orders Vic to train him. If he does, she tells him, he might not be forced out. In their first case together, they search for a gang revolutionary who's wanted by a Farmington gang lord. Meanwhile, Aceveda makes a deal with a developer that lands the San Marcos murders back in the Barn, and Dutch is surprised to hear that Tina has transferred. He's more surprised to see her on an LAPD publicity poster.

Shane's wife discovers his affair with Tilli, while Vic gets closer to the truth about Lem's murder. Elsewhere, Julien joins the Strike Team---now led by Kevin Hiatt ---as Claudette assigns it the San Marcos murders; the father of a teen runaway offers to help Dutch and Billings hunt for the child rapist they're after; and a disgraced ex-cop (Carl Weathers) asks Vic for a favor.

After learning that Guardo didn't kill Lem, Vic first turns his suspicions toward Antwon, then closer to home. Elsewhere, Guardo's girlfriend (Lela Loren) tells her story to Dutch while Vic finds a way to use the murder of a politician's daughter to his advantage; Tina returns to the Barn, and Dutch notices that she notices Hiatt; and Vic's daughter Cassidy (Autumn Chiklis) meets her new stepbrother. She's not happy about it.

Shane reaches out to the Armenian mob after Vic cuts him off. Meanwhile, Vic gets more involved in the San Marcos massacre after discovering evidence of a larger conspiracy, and Dutch and Billings make headway of their own on the case when they track down the lone survivor of the slaughter.

Hiatt tells Vic about Claudette's double-dealing, and disobeys an order she gives him after Hernan turns up missing; Shane freelances for the daughter (Franka Potente) of an Armenian mob boss; Danny, Tina and Corinne go shopping for upscale accessories in a bad neighborhood; and Ronnie learns the truth about Terry Crowley's murder.

As the Strike Team makes progress with the San Marcos murders, Vic gets bad news about his job prospects---then finds an ally he didn't expect. Meanwhile, Shane and Diro become business partners as she prepares to take over the Armenian mob; and Billings meddles in Tina and Hiatt's love lives.