On his first day, rookie Ben Sherman begins to wonder if being a cop in Los Angeles is too much for him to handle. The pressure mounts when he is assigned to veteran John Cooper, a no-nonsense training officer. Meanwhile, a young girl is kidnapped and Detectives Adams and Clarke investigate.
Cooper, Sherman and Brown find a baby that has been abandoned in the middle of a busy intersection. The case hits home for Adams, who must deal with her past in order to help the baby. Meanwhile, bryant and Moretta work to find enough money to put a teenager under police protection.
When Sherman talks about his past to the department therapist and Cooper, everyone begins to realize his motivation to become a police officer. Meanwhile, Dewey, star-struck by Timmy, a self-obsessed actor, decides to party with him rather than book him. Trouble ensues when Dewey loses Timmy, causing Chickie to wonder whether or not she should be partners with Dewey.
Detectives Moretta and Bryant help Salinger when he crashes his car into a light pole and a gang member steals his gun. Meanwhile, a man exposes himself to Detective Adams.
Detectives Bryant and Moretta are called to an alley to investigate when an unidentified female was used by a gang as target practice. Meanwhile, Lydia upsets Detective Clark when she believes he got too personal on their latest case.
As Detective Adams and Detective Clarke look into a string of jewelry thefts in Bel Air, Officer Sherman becomes personally involved in the case when he learns one of the victims is someone he knows. Meanwhile, Detective Bryant and Detective Moretta get Janilla ready to testify at the trial and must use any means necessary to keep her safe.
The detectives investigate when a Marta's Avenue gang member is found murdered. Meanwhile, Detective Bryant tries to convince Lydia to help Janilla; and Detective Salinger and his daughter continue to have relationship problems.
The possible abduction of an elderly man by a known drug addict; a freeway shooting; Chickie's new partner shoots a 13-year-old boy.
Ben and John respond to a triple homicide; Lydia and her new partner realize they have different styles of investigating; Sammy continues to have problems with his wife.
Ben winds up in a patrol unit alone for the first time. Among the cases he has to handle on his own are a missing girl and a suspected stalking case. Lydia and Rene spend the day checking in on and helping out Brianna and her baby. Over in Gangs, Gil Puente leads the task force to take down ex-banger Trinny Day, but Sal and Sammy have a hard time working with Puente. While patrolling with John, it is clear that Chickie has no command presence, but she is determined to work harder and become a good cop again.
Lydia and Rene arrive in Compton, where a young USC student was murdered. Kenny and Andy canvass the neighborhood while Nate and Sammy investigate a house nearby where a party took place the night before. John and Ben head to Indian Wells for the funeral of a cop that proves to be very personal for John. They also stop at a halfway house for cops to see how Dewey is doing. And Lydia decides to talk to her mother about something that has been festering between them for years.
Gangsters open fire in a crowded club; tensions rise between Sal and Gil; Sammy confronts his wife about photographing gang members.
John gets an unexpected request from his ex-wife; a maximum deployment is used to capture a suspect who has been impersonating a cop; Lydia is partnered with Russell for a double homicide; Sal and Susan celebrate her promotion.
Lydia and her new partner hunt a cleaning lady's killer. John and Ben rescue an officer during a bank robbery shoot-out. Nate and Sammy investigate the murder of a gang member.
Sammy and Nate feel threatened when Detective Daniel "Sal" Salinger calls in Lydia and Josie to help them solve a series of gang murders. John gets saddled with Officer Dewey Dudek in order to keep an eye on him after his stint in rehab. And Ben rides with Officer Chickie Brown who quizzes him about his relationship with a woman obsessed with cops.
Ben seeks revenge when his mother's rapist is released from prison. Lydia and Josie bump heads over a chainsaw, as Sammy deals with his wife's betrayal.
John teaches Ben several key tips to being a police officer, including the proper ways to make a death notification and arrest a suspected drug user. Sammy stays with Nate's family while he sorts out how his marital problems. Lydia and Josie handle a drug-related murder case with an unorthodox suspect.
Lydia and Josie catch a complicated rape case on the Westside. John bonds with an abandoned teenager while also having to deal with testifying at his own father's parole hearing.
While working a celebrity murder, Lydia discovers the pros and cons of being in the spotlight. Sammy leaves the confines of the office to go out on patrol. John and Ben chafe with frustration over a tedious security detail.
Lydia gets suspended from duty and bonds with an injured witness. A high-speed chase haunts Chickie, and John tries to clean up a neighborhood.
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.
Cooper's drug problem becomes worsen as Ben nears the end of his training period.
Ben faces his final day as a rookie and contends with his partner's increasing dependence on painkillers. Meanwhile, Sammy is assigned a new partner.
In the Season Four premiere, Officer John Cooper returns to duty after recovering from back surgery and meets his new partner, Officer Jessica Tang (special guest star Lucy Liu), a tough and disciplined cop with her own set of baggage from working the streets. Lydia, whose new partner is Ruben Robinson (guest star Dorian Missick), can't seem to escape helping a former CI who won't stay in safety. Ben, Sammy and Dewey chase an armed man into an elementary school, while Ben bumps heads with an intense street cop (guest star Lou Diamond Phillips) whose cynical attitude is affecting their work.
Tang and Cooper’s past physical problems are put to the test when they are attacked by a perp on the street. 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. Ben and Sammy ride with another officer who is prone to small mistakes. And Joel Rucker takes over as the new captain.
It's Hood Day on the streets of Los Angeles, and Captain Rucker (guest star Carl Lumbly) has the department playing hall monitors. Sammy lets old grudges die hard when he leaves a gang territory unattended, only to have a violent gang war erupt throughout the city. Ben's recent mistake continues to haunt him publicly everywhere he goes. Ruben (guest star Dorian Missick) practices his speech for his daughter's quinceañera in the midst of a murder investigation. Cooper and Tang investigate a robbery in a strict Orthodox Jewish home, leading Tang to reveal parts of her past.
Tang's past comes to the surface when she offers help to a homeless veteran. Elsewhere, Sammy cares for a dog after accidentally shooting it; Lydia and Ruben investigate the murder of a good Samaritan; Ben and Lydia brave monumental life changes.
A case turns personal for John when he tries to save a suicidal teen; Sammy's attempt to transform a former drug addict into an informant comes back to haunt him; a person from Tang's past arrives; and Lydia and Ruben scrutinize a home invasion.
A young man set to testify in one of Sammy's cases is killed; John and Tang enter the reel world when they serve and protect a documentary-film crew; and Lydia's body is tested when she works as a shift supervisor.
The day just goes from bad to worse when Tang (special guest star Lucy Liu) is served legal papers in the morning and involved in a shooting in the evening. Sammy holds a grudge against Ben for not backing him up over drug evidence that may have been planted during a recent arrest. And Lydia comes to a decision about how to deal with her pregnancy
Lydia and Ruben probe the death of a nanny. In an alarming investigative snag, the children who were in her care have gone missing. Meanwhile, Ben's efforts to help a prostitute with a daughter backfire; and John corners Tang about coming clean about a past shooting.
Ben tries to help the daughter of a prostitute, only to draw the wrath of the woman's pimp. Meanwhile, Lydia and Ruben snoop into the murder of a gang member; and Tang faces the mounting consequences of shooting an innocent bystander.
It's the last day on patrol for Tang (special guest star Lucy Liu), 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 Danny 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.