Police officers Brian Taylor (Jake Gyllenhall) and Mike Zavala (Michael Peña) are close friends and partners in the Los Angeles Police Department. Taylor is filming their police activities for a film project, attaching small cameras to his and Zavala's uniforms and carrying around a camcorder, much to the dismay of their peers and superiors. After shooting two suspects following a high speed chase, the shooting is declared justified by the D.A. and the officers are commended for their actions. Taylor and Zavala's antics are met with scorn by fellow officer Van Hauser (David Harbour).The officers respond to a call regarding a man, Mr. Tre (Cle Shaheed Sloan) scaring off a mailman while intoxicated. Upon arrival, Tre hurls racist insults at the Hispanic Zavala, who responds by accepting a fight, to Taylor's approval. Zavala beats him soundly and arrests him, but wins the man's respect for both fighting fair and not mentioning the fight in the report (which may have sent Tre to prison for life due to Three Strikes). Later that night, Tre and his friends are shot at by a group of Latino gang members and one of his friends is killed. The officers find the now-burnt vehicle used in the drive-by the next day, but are shooed off the scene by homicide detectives as well as Van Hauser, who warns them they will get screwed over by the LAPD someday.Responding to a missing persons report, the officers discover two children bound and locked away in a closet and arrest the distressed mother and her drug-addict husband.The cops arrive as a group at a loud outdoor party, the Hispanics from the drive by shooting are celebrating. The cops and partiers verbally joust and Brian sizes up the gang leader, Big Evil (Maurice Compte). The cops depart telling them to keep the noise down.Taylor begins dating Janet (Anna Kendrick) and feels Janet is the only girl he's dated who can connect with him on an intellectual level. Zavala, who is married and expecting a child, offers him relationship advice.Investigating the South Central area, Taylor has a hunch and determines to scope out the home of the mother of a known gang member and sees an expensive pick up truck in the driveway. A woman walks out of the house and delivers a large soup saucepan with lid to the man in the truck. As the truck drives away, the officers follow and then pull over the man in the truck, ostensibly for having CD hanging off the mirror and illegally obstructing the front view, which is minor traffic violation in California per VC 26708 . As Zavala approaches the driver's window to make contact, the driver suddenly draws a gun and fires, and Zavala deflects the man's arm just in time to avoid being shot. After arresting the driver, the officers find an ornately-decorated, compact .45 Colt automatic pistol, a gold-plated AK-47 rifle, and a large amount of money in his truck (hidden in the soup pot). As revealed later, the money and firearms are connected to a Mexican drug cartel operation in the South Central area as well as the gang that did the drive-by on Tre and others.Days later, the two officers are first responders at a house fire that has trapped children on the second floor. Zavala and Taylor enter the house and rescue two young children and are nearly overcome by heat and smoke. When the mother informs them that a third child remains inside, Zavala to rushes to the aid of the remaining child as Taylor reluctantly follows, barely making it out alive. The two are commended and receive the Medal of Valor for their actions, but Taylor has mixed emotions about the situation. Using the house fire incident as leverage, Taylor convinces Zavala to further investigate the South Central incident, to Zavala's chagrin. Arriving at the house, Taylor and Zavala notice suspicious behavior from outside and enter. They arrest another man, who is also in possession of several ornate firearms: a .45 Colt automatic similar to the first one found at the traffic stop, and a Walther PPK. Investigating further, Taylor discovers a hidden stash of Mexican and Asian prisoners, indicating that they have just stumbled upon a human trafficking case. Upon exiting the building, they are accosted by the suddenly arrived federal officers of the ICE department, informed that the man had been a person of interest with possible leads to the cartel, and strongly urged to "lay low" due to possible cartel reprisals. Taylor is left confused and agitated.Soon after, Zavala's wife Gabby (Natalie Martinez) gives birth to the couple's first child, Mike Zavala Jr.One night, the two receive an 'officer needs help' call from Sook (Kristy Wu), Van Hauser's probationary partner. She fails to give the address and is unable to communicate their location effectively before she screams and the call is cut off. Urgently responding to the call, the two find Van Hauser calmly waiting in front of a building with a knife stabbed into his right eye, warning the two of a large criminal around the corner. Taylor and Zavala grab a shotgun from their car and investigate, finding the man brutally beating Officer Sook. Instead of using deadly force, Taylor orders the suspect to stop, who, surprisingly, immediately stops the assault and surrenders without any sort of resistance. The suspect is detained by Zavala, while Taylor calls for an ambulance and attends to the rookie officer, whose face has been badly injured. The Sarge, wonders why Brian didn't kill the man, and mentions Sook turned in her badge, resigning from the Dept.Taylor later marries Janet and, after a night of celebration, Zavala drunkenly, though earnestly, tells Taylor that, should anything happen to him, he will take care of her.After deciding to respond to a more easygoing call the next day, the officers go to investigate a welfare check from an elderly woman. After receiving no response, the officers break down the door and discover the dead homeowner along with a number of dismembered corpses, tortured, and killed by the cartel.Elsewhere, an unknown observer (heavily implied to be someone from an unnamed federal agency) records footage of a cartel member speaking on the phone to put a kill order on the officers, and the LA gangsters from the earlier drive-by begin plotting their assassination. While Taylor and Zavala receive no warning from the LAPD or the federal agency that recorded the conversation, Tre, having a higher level of respect for Zavala after the earlier fight, warns them of rumors that they are now hot targets for the cartel, but they disregard his comments, saying "We're cops, everyone wants to kill us."The two cops are followed by Big Evil and his gang, they realize they need a good plan.Janet becomes pregnant shortly after the marriage.After a short pursuit with a reckless minivan one night, the officers chase the driver into an apartment complex, where the gangsters have set up an ambush to kill them. The officers are fired upon with AK-47 assault rifles and Officer Taylor is shot in the hand, destroying his radio. Taking refuge in a small apartment, Taylor decides that they are going to have to gun their way out. The two open fire, suppressing the gunmen, and end up killing a shooter on the way out of the complex. Awaiting backup outside, the two are fired upon once more and Taylor is shot in the chest. Zavala kills the shooter, desperately attends to Taylor, and cries out for backup, but Taylor remains unresponsive. Zavala begins to believe Taylor is dying and cries in mourning, while frantically calling for backup. Realizing that the gangsters from inside the apartment have approached behind him, Zavala reaches for his firearm, but is repeatedly shot in the back. As a last-ditch effort to save his partner, he throws his body over the seriously wounded Taylor, protecting him from further injury, at the cost of his own life. Backup eventually arrives and a brief shootout ensues. The gangsters, including "Big Evil", are shot and killed when they refuse to drop their weapons and spray shots at the backup officers.Zavala is killed in his effort to save Taylor, but Taylor miraculously survived the encounter, albeit heavily injured. A funeral is held for Officer Zavala, during which Taylor tearfully declares that Zavala was his brother, before breaking down in tears among his wife and Zavala's family. The screen fades to black. In an epilogue scene, a clip is shown from earlier in the day of the shooting, during which Zavala details hiding under his then-newlywed wife's parents' bed while the parents had sex. Taylor and Zavala both laugh heartily, before going off to "fight crime or some shit".