Lydia endures a stressful adjustment period with her new partner, Detective Josie Ochoa, as they investigate the disappearance of an illegal immigrant. Meanwhile, Russell must learn to adapt to a new desk job, Sammy and Nate look for the killer of two gangbangers, Ben catches the eye of an attractive woman, and John struggles with increasing back pain.

Lydia and Josie are assigned to assist Sammy and Nate with a series of gang murders. John discovers he can't stand the "new and improved" Dewey, fresh from rehab, when he's forced to ride with him in order to evaluate whether Dewey is competent to return to duty. Ben must then ride with Chickie, who drives him nuts with her very personal questions about his private life and his new relationship with the "cop groupie" he's been seeing.

Ben has a confrontation with his mother’s rapist after the man is released from prison and is devastated when his mom reveals some information she's been keeping secret for years. Meanwhile, Lydia and Josie continue to bump heads, and Sammy deals with his wife’s betrayal.

Ben learns the hard way, some valuable lessons about the importance of following established procedure and protocol, Lydia's current murder investigation takes an unexpected turn, and Nate and Sammy are unprepared when a group of people they encounter turns into a violent mob.

While working a celebrity murder, Lydia discovers the pros and cons of being in the spotlight. Meanwhile, six weeks after Nate's murder, Sammy finally returns to street patrol. Elsewhere, John voices his anger over the department's waste of money, man-power, and hours spent guarding the home of a celebrity suspected of murdering his estranged wife from the reporters trying to get a scoop.

When crime scene photos from the case of the celebrity suspected of murdering his estranged wife show up on the internet, all of the first responders are suspended, including Lydia and Josie. Further investigation proves the photos were the unauthorized pictures taken by Lydia using her phone, and she's shocked when she learns who was responsible for the leak. Meanwhile, Sammy fails to pick the right guy from the line-up for Nate's murderer, and Dewey and Chickie are involved in a high-speed chase that ends in disaster.

Sammy seeks answers in the desert, Lydia and Josie babysit an important alcoholic witness before a murder trial, and John and Ben go against procedure to help a child in need.

As John's back pain worsens, so does his drug problem, and Ben, about to finish his training period, is gravely concerned about him. Meanwhile, when a man wielding an automatic weapon goes on a killing spree inside a factory, Josie gets injured and Lydia must go after him alone. Elsewhere, Sammy decides to take a more active roll in the upcoming birth of his child..

On Ben's final day as a rookie, he decides to force John to get the surgery he needs to relieve his back pain which, in turn, will end his need for painkillers. Meanwhile, Josie discovers that her son is the man Lydia has been seeing. Elsewhere, Sammy decides to go back to regular patrol.