A rundown neighborhood in South Central Los Angeles, known as the “Anderson ghetto”, is filled with street gangs and thugs. Members of a local gang, called Street Thunder, have recently stolen a large number of automatic rifles. Consequently, the L.A.P.D. sends a team of heavily armed officers to ambush and kill six members of the gang. Following this attack, the four gang warlords swear a blood oath of revenge, known as a “Cholo”, against the police and the citizens of Los Angeles.

On the same day that the Street Thunder members swear revenge, one of the Anderson police stations, Precinct 9, Division 13, is about to close down. The old police station is running on a skeleton staff composed only of officer Chaney and the station's two secretaries, Leigh (Laurie Zimmer) and Julie (Nancy Loomis). Lieutenant Ethan Bishop (Austin Stoker), a newly promoted California Highway Patrol officer, is assigned to run the station for its last few hours of service, even though this assignment falls outside the usual scope of his duties as a CHP officer.

Meanwhile, a man named Lawson (Martin West) is driving through a nearby area with his young daughter, Kathy (Kim Richards), searching for the street where her nanny lives. Frustrated, he stops to call the nanny at a public telephone booth. While Lawson is on the phone, his daughter goes to buy an ice-cream from a nearby truck. Unseen, the Street Thunder warlords approach, and one of them shoots Kathy and the ice-cream vendor, killing the little girl instantly and fatally wounding the man. With his dying breath, the ice-cream vendor tells a distraught Lawson that there is a gun under the ice-cream truck's dashboard. Taking the gun, Lawson pursues the four gang leaders into the Anderson area and kills the one who shot his daughter, who by chance is one of the gangs warlords. At a loss for what to do next, he goes to a nearby phone booth, but then sees he is being pursued by the remaining warlords and other gang members. Seeking shelter, he runs into the nearly deserted police station. In shock, Lawson cannot communicate what has happened to him.

Around the same time, three prisoners are being transferred by bus from one penitentiary to another facility several hours away, when one prisoner becomes increasingly ill. Starker (Charles Cyphers), the officer in charge of transporting the prisoners, decides to get medical assistance. He tells the bus driver to pull over at the nearest safe location, which happens to be Bishop's station. The prisoners, Napoleon Wilson (Darwin Joston), a convicted killer on his way to Death Row, Wells (Tony Burton), and the sick Caudell (Peter Frankland), are put into the holding cells while Starker attempts to call a doctor. The telephone lines suddenly go dead, and Starker, frustrated by his inability to get help, prepares to put the prisoners back on the bus.

The street gang, armed with heavy weapons such as the Colt M16, suddenly open fire on the precinct. In a matter of seconds, they kill Chaney (Henry Brandon), the bus driver, Caudell, Starker, and the two uniformed officers assigned to support Starker during the prisoner transfer. While bullets fly, Wells rushes back into the station and Bishop rescues Wilson, who is chained to (and trapped under) Starker's body. Bishop then puts Wilson and Wells back into the holding cells.

The gang, having already cut the telephone lines, then cut the station's electricity. the three remaining warlords come forward to break a blood-filled bowl symbolizing the Cholo on the precinct's front steps, formally signifying that a siege is underway. Heavily outnumbered, without options, and once again under attack as the gang members resume shooting, Bishop sends Leigh down to the holding cells to release Wells and Wilson, who then take up arms. A battle rages as gang members launch a suicidal attack on the station in a crazed attempt to kill all inside. Wilson, Bishop, Wells, and Leigh successfully defend the station, but Julie is gunned down in the ensuing chaos.

After the gun battle, the gang members quickly remove all evidence of the skirmish and appear to momentarily retreat. Wilson, Bishop, Leigh, and Wells then decide that one person should try to sneak out of the station, hot-wire a car outside, and drive away to summon help. Since Leigh has a wounded arm, and Bishop does not know how to hot-wire a car, Wilson and Wells play a game of “potatoes” to decide who will go. Wells loses. Although Wells appears to accomplish his task, he is shot and killed by a gang member hiding in the back seat of the car.

As the gang rally for a third time, Wilson, Leigh, and Bishop go down to the basement, taking the still-catatonic Lawson with them, and stage a “last stand”. This culminates in Bishop shooting an acetylene tank, which explodes violently, killing the gangsters inside the station. Police back-up arrives and secures the station, relieving the four survivors of the siege: Bishop, Leigh, Wilson, and Lawson.