Politically ambitious new LAPD captain David Aceveda tries to crack corruption in the Barn by recruiting Terry Crowley to help him take down fellow detective Vic Mackey, leader of a police Strike Team consisting of himself, Shane Vendrell, Curtis "Lem" Lemansky, and Ronnie Gardocki. Meanwhile, detectives Dutch Wagenbach and Claudette Wyms investigate a woman's murder only to discover her missing daughter has been sold off by her addict father to a child molester.

The Internal Affairs Division begins an investigation into the shooting and death of Terry, while Aceveda starts his own. Meanwhile, Dutch and Claudette investigate the shooting of a churro vendor that ends in the Strike Team busting up a gang initiation.

Mackey attempts to broker a peace agreement between rival rappers whose feud is spilling into the streets. Danny is attacked after identifying a shooter, Julien helps a woman whose son's body has been stolen, and Dutch and Claudette search for a missing migrant worker.

Aceveda insists that Vic take uniformed cops in with them to bust an Armenian gang's drug delivery. After the bust, Julien sees the Strike Team take a portion of the drugs for themselves.

When the body of a young hooker is found in the district, Dutch and Claudette get the full resources of the Barn for 24 hours and the help of the FBI to search for Dutch's serial killer.

Vic's relationship with Shane becomes even more complicated when the pair hook up with a cop from another precinct to investigate a shooting. Meanwhile, Dutch and Claudette bring in a psychic on a complaint from the son of one of her clients.

Allegations against the Strike Team are front-page news, and the team is broken up pending the investigation. Vic and Claudette find themselves following the trail of a meth batch, while Shane and Dutch attempt to get along on a case.

A planted gun in a truck-jacking case causes complications for the Strike Team, and they're left to clean up their own mess. Meanwhile, the rift between Danny and Julien deepens, causing Aceveda to intervene, and Claudette deals with the mess of her family during a visit from her daughter.

Vic assists Connie as she tries to quit crack cold-turkey in a desperate attempt to keep her son, leaving Shane and Lem to investigate a strip-club mugging ring. Meanwhile, Dutch follows his instincts on the serial killer case and Danny is bitten by an HIV-infected perpetrator.

Vic and the Strike Team mediate a dispute between Rondell Robinson and the Nation of Islam while Dutch and Claudette investigate a brutal home invasion in the Korean community.

Aceveda turns to Claudette after a double homicide raises ire and fear within an ignored community. Vic is forced to help Assistant Chief Ben Gilroy avoid being implicated in a hit-and-run, while Shane and Lem follow the trail of a gunrunner with a penchant for cockfights.

In the aftermath of a riot, cops are being lured into ambush shootings by a rash of bogus 911 calls. Vic and Aceveda are forced to work together to stop the attacks and solve the problems in the Grove, while Gilroy tries to pit the two against each other in an attempt to save his own job.