An investigation is opened after Ben, John, and Officer Chickie Brown encounter an infant crawling down the street. Lydia takes a personal interest in the case after Child Protective Services gets involved, and goes out on a limb for the mother. Meanwhile, Sammy and Nate are furious when they learn their teenage eyewitness won't get police protection due to lack of funds, and put up their own money after an attempt is made on her life.

In an effort to recover Detective Daniel "Sal" Salinger's stolen weapon, Sammy and Nate crack down on the residents in the neighborhood where it occurred. Elsewhere, a man exposes himself to Lydia.

A young Jane Doe found murdered in an alley sets Lydia on an emotional journey, and she discovers her suspect was involved in a cold case. Meanwhile, Sammy's wife, Tammi, decides to embark on a new career, and Nate deals with some family issues and reveals a deep secret.

On his first day, rookie officer Ben Sherman is assigned to veteran cop John Cooper, his no-nonsense training officer. After being involved in a fatal shooting, Ben already begins questioning his career choice. Detectives Sammy Bryant and Nate Moretta try to convince a young teen to testify after learning she is an eyewitness in the attempted murder of a boy who was mistaken for a gang member. Meanwhile, Detective Lydia Adams investigates the disappearance of a young girl.

Ben and John are called to the home of a man who wants them to find his dog, and another in which a drug overdose led to homicide. Chickie and her partner, Officer Bill "Dewey" Dudek, butt heads after he makes some poor decisions that may have repercussions, and later, Dewey finds himself in an embarrassing situation.

John copes with the aftermath of his kidnapping. Sammy closes in on the real robbers. Lydia continues to pursue John’s kidnappers and becomes more intimate with Russell.

Lydia and her partner, Detective Russell Clarke, investigate a string of jewelry thefts in Bel Air and Ben discovers one of the victims is someone he knows. In another matter, Ben must face his past when it comes back to haunt him. Meanwhile, John reveals a secret he's been harboring for a long time, Russell has troubles at home, and Sammy and Nate take extraordinary steps to protect their eyewitness, Janilla, after another attempt is made on her life.

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.

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..

Lydia and Ruben investigate the death of a young nanny, but first they have to find the kids who were in her care. Ben tries to intervene in the life of a prostitute with a daughter, but his efforts only make things worse. Tang gets an update about the boy she shot, but John still wants her to come clean about what really happened.

Officer John Cooper clashes with his new boot, Gary Steele, a vet from Afghanistan. Lydia struggles to balance detective work and motherhood. Ben Sherman falls in with a sketchy crowd of cops, while Sammy Bryant is trapped in a hellish custody battle with Tammi.

Lydia and her new partner, Detective Rene Cordero, investigate the disappearance of an elderly man. Meanwhile, Nate and Sammy investigate a freeway shooting and inadvertently threaten the integrity of a major undercover operation. Elsewhere, Chickie finds herself in the middle of a mini-riot after her new partner panics and takes a shot at a 13-year-old boy.

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.

When a rape victim hires a lawyer and refuses to talk with the two detectives unless her lawyer is present, Lydia and Josie want to know why. Meanwhile, John strikes up an unlikely friendship with a boy who returned home after a sleepover, only to discover his parents had packed up and moved during the night, abandoning him and leaving him to fend for himself, Later on, John begins to feel the stress over the prospect of testifying at his father's upcoming parole board hearing, but he doesn't realize his testifying will open up an emotional can of worms that he's not prepared for.

Things are gonna change with Joel Rucker as the new captain. Lydia and Ruben investigate a convenience store murder, and Lydia uses her typical compassionate charm to collect facts on her suspect from an unassuming grandmother. Meanwhile, John and Jessica get to test their physical abilities when they're attacked on the street. Elsewhere, Sammy and Ben ride with a mistake-prone officer, but Ben is the one making mistakes... mistakes that earn him some notoriety.

It's the last day on patrol for Tang, as she prepares to move to another precinct as sergeant. But first she wants to tie up some loose ends on the beat. Lydia is pressured to make a decision about the extent of her police duties during the rest of her pregnancy. With Sammy recovering from injuries, Ben and Dewey are partnered together. But Ben is bent on tracking down the prostitute he's been trying to help and then getting her and her daughter away from the abusive pimp controlling their lives.

After Mendoza is injured in a drive-by, Ben takes it upon himself to solve the case, much to Sammy's dismay. Lydia regains her footing as a detective while investigating the murder of a female boxer's husband. Tensions between Cooper and Steele finally come to a head during a traffic stop-gone-sideways.

A breakdown in the LAPD's dispatch system makes a difficult job even harder for the cops, all of whom are already grappling with communication issues. Lydia, in a moment of weakness, reaches out to her old flame, Terrell. John is assigned a new partner; Ben and Sammy's partnership is strained just as Ben makes a romantic connection on the job.

Lydia and Reuben track the killer of a man whose body was slow cooked down to the bone. John and Lucero settle into their new friendship during a day filled with strange calls. Ben and Sammy respond to an insane bank robbery in which the suspects throw cash out into the streets as they make their escape in a getaway car.

John is forced to think about what life will hold for him once he leaves the LAPD. Lydia connects with a man she sent to death row earlier in her career. Much to his chagrin, Sammy becomes a tabloid celebrity for the day after a very public act of heroism. Ben makes a dubious decision to get a drug dealer arrested. And a foot pursuit puts Dewey in danger.

The day’s events bring all of our cops face to face with some of their greatest fears. The death of a baby shakes Lydia to her core. Sammy is questioned by I.A. about his altercation with Tammi. A difficult day on the job causes Cooper to seek solace in Hicks but finds none. Ben becomes more deeply involved with Brooke while continuing to see Elena.

When Sammy discovers Marta has made bail, and fearing another attempt will be made on Janilla's life, he arranges to have her stay with Lydia, but Janilla's safety is compromised after she visits her old neighborhood. Meanwhile, Sal continues to spy on his daughter's activities, Dewey flips out when Chickie confronts him about his drinking problem, and Russell is caught in the middle when he attempts to defuse an ongoing domestic battle between a neighbor and his wife.

John and Ben are the first responders to a gruesome triple homicide in Hancock Park, where a mother is bludgeoned to death and her young daughters are found bound, gagged, raped and dead in their bedrooms. In investigating the case, Lydia and Rene realize they have two very different investigation styles. Meanwhile, Sammy continues to have problems with his wife and Ben confronts John about his increasing reliance on pain pills.

Lydia and Rene are called to Compton to investigate the murder of a young SULA student, only to discover that the girl was not exactly who her parents claimed she was. Meanwhile, John takes Ben with him to Indian Wells to attend the funeral of his friend, a cop who committed suicide. On the way back, they stop at the rehab center to check in on Dewey.

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.