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.

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.

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.

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.

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.

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.

Ben, assigned to work on his own for the first time, responds to a call about a 9-year old girl who wandered off and finds himself emotionally distraught over a woman who is being harassed by an ex-lover with a tendency towards violence against women. Lydia and Rene spend the day checking assisting Brianna, the mother of the baby found crawling down the street. Over in Gangs, Detective Gil Puente leads the task force to take down ex-banger Trinny Day, but clashes with Sal and Sammy who find him loud and abrasive. When Chickie is assigned to ride with John for the day, he discovers she has no "command presence" and has lost her edge, suggesting it may be time for her to get out of police work.

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.

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.

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.

John returns to duty after recuperating from back surgery and meets his new partner, Officer Jessica Tang, a tough and disciplined cop with her own set of baggage from working the streets. Lydia, whose new partner is Detective Ruben Robinson, can't seem to escape helping a former CI who seems unable to stay out of danger, Ben, Sammy and Dewey chase an armed man into an elementary school, and Ben bumps heads with an intense street cop whose cynical attitude is affecting their work.

Lydia dons a uniform and hits the streets when the shift is short a supervisor and she must fill in. Meanwhile, John and Jessica are stuck with a film crew tagging along on their calls. Elsewhere, Ben infuriates Sammy when he implies Sammy planted evidence.

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.