The story takes place on a Saturday day and night in Anderson, a crime-infested ghetto in South Central Los Angeles. Members of a local gang, called 'Street Thunder', have recently stolen a large number of automatic rifles and pistols. The film begins the night before, as a team of heavily armed LAPD officers ambush and kill six members of the gang. In the morning, the gang's four warlords swear a blood oath of revenge, known as a "Cholo", against the police and the citizens of Los Angeles. During the day, three sequences of events occur in parallel in and around Anderson: First, Lieutenant Ethan Bishop (Austin Stoker), a newly promoted CHP officer, is assigned to command the old Anderson police precinct during the last few hours before it is permanently closed.
The station is manned by a skeleton staff composed of one officer, Chaney (Henry Brandon), and the station's two secretaries, Leigh (Laurie Zimmer) and Julie (Nancy Loomis). Across town, two of the Street Thunder warlords with two other gang members drive around the neighborhood looking for people to kill. One of the warlords, shoots and kills a little girl and the driver of an ice-cream truck. The girl's father, Lawson (Martin West), pursues and kills the hoodlum, whose fellow gang members chase the man into the Anderson precinct. In shock, he is unable to communicate to the officers what has happened to him. In the third part, a prison bus commanded by Starker (Charles Cyphers) stops at the station to find medical help for one of the three prisoners being transported to Death Row at the state prison. The prisoners are Napoleon Wilson (Darwin Joston), a convicted violent killer on his way to Death Row, Wells (Tony Burton), and Caudell, who is sick. As the prisoners are put into jail cells, the telephone lines go dead, and when Starker prepares to put the prisoners back on the bus, the gang opens fire on the precinct, using weapons fitted with silencers.
In seconds, they kill Chaney, the bus driver, Caudell, Starker, and the two officers along with Starker. Bishop unchains Wilson from Starker's body and puts Wilson and Wells back into the cells. When the hoodlums cut the station's electricity and begin a second wave of shooting, Bishop sends Leigh to release Wells and Wilson, and they help Bishop and Leigh defend the station. Julie is killed in the firefight, and Wells is killed after being chosen to sneak out of the precinct through the sewer line.
Meanwhile, the gang members remove all evidence of the skirmish in order to avoid attracting outside attention. Bishop hopes that someone has heard the unsilenced police weapons firing, but the neighborhood is too sparsely populated for nearby residents to pinpoint the location of the noise. As the gang rallies for a third assault, Wilson, Leigh, and Bishop retreat to the basement, taking the still-catatonic Lawson with them. The gang then storms the building in an all-out final assault and confront the survivors in the basement. Bishop shoots a tank full of acetylene gas, which explodes violently, killing the all hoodlums in the basement.
Finally, two police officers in a cruiser radio for backup after discovering the dead body of a telephone repairman hanging from a pole near the police station. When more police officers arrive and secure the station, the surviving gang members retreat. Venturing down into the basement, the police officers find dozens of dead and badly burned gang members strewn in the hallway, and the only survivors are Bishop, Leigh, Wilson, and the comatose Lawson..