March 21, 2023: as the hours count down to the annual Purge, the news credits the national holiday which permits all crimes, up to and including murder, for twelve hours with reducing unemployment and poverty to historic lows. Resistance groups, however, hijack television broadcasts in order to inform the public that the real reason of the Purge was not to act as catharsis for the American public, but as a means of population control, with the impoverished being targeted most.
A Los Angeles waitress named Eva Sanchez rushes home to barricade herself, her daughter Cali, and her ill father Papa Rico. While they barricade their home, however, Rico slips out, leaving a note that states they would receive $100,000 in exchange for him giving himself up to a group of wealthy Purge enthusiasts.
Meanwhile, a married couple, Shane and Liz, are driving to a family member's house to wait out the Purge. Unfortunately, their car has been sabotaged and breaks down at the stroke of 7:00 PM, the Purge commencing with them far from safety. The saboteurs, a gang of bikers, chase Shane and Liz through the streets while a former police officer makes plans to carry out revenge for the death of his son, using the Purge as his opportunity to kill his child's killer.
At Eva's apartment, the building's superintended breaks into her apartment. Having been spurned by Eva in the past, he plans to rape and kill both Eva and Cali. He is shot and killed by a group of paramilitary soldiers, who then abduct Eva and Cali, who plan to have them killed by their leader, a chaingun-toting madman known as "Big Daddy". Thankfully, Eva and Cali are rescued by the former police officer, who dispatches the paramilitary men and incapacitates Big Daddy. He takes the two with him, along with Shane and Liz, who hid in the back of his car, as he escapes from Big Daddy's wrath, his car being heavily damaged and breaking down a few blocks down. Eva has a car a co-worker lent her at her apartment, and offers to let the officer use it if he can escort them back to her apartment.
On their way to the apartment, they find a van surrounded by several dead paramilitary men, with many weapons kept inside. Taking the weapons, they make their way into the subway tunnels, where the homeless stay in order to keep safe during the Purge. Unfortunately, it's not safe here either, as Purgers wielding miniguns and flamethrowers pursue them, killing the homeless along the way. Shane and Liz use the officer's machine guns to fight back, setting off a flamethrower's tank and sending the killers up in flames as they take to the surface once more.
The group make it to the apartment, only to discover the co-worker Tanya's car is gone. They take shelter with Tanya and her sister Lorraine. The shelter proves to be anything but harmonious, however, as Lorraine kills Tanya over sleeping with her husband, and chases Eva and her companions out. They end up being pursued once more by Big Daddy, who tracked them on traffic cameras, but they manage to evade him, only to get abducted by the bikers pursuing Shane and Liz.
The bikers take Shane, Liz, and the others to a building, explaining that they won't kill them, but they will still die that night. They are then delivered to a group of wealthy Purgers, who gamble on their survival in a dark room pursued by men with night-vision goggles and guns. The officer manages to gain the upper hand on their would-be killers, and the bluebloods send elite security officers to dispatch them, with Shane dying in the ensuing gun fight. The security detail is taken out, though, by insurgents who oppose the NFAA and the Purge. Liz decides to join up with the insurgents to avenge Shane, while the officer escapes in one of the bluebloods's car, along with Eva and Cali.
As morning dawns, the officer comes upon the house of the man he was looking for: Warren Grass, who had been driving under the influence when he hit the officer's son. Even though Grass was remorseful for the death, the officer pursued him because he got off with a light charge. He attacks him in his bedroom, holding him at knife point, and threatening to kill him.
He later leaves and is ambushed by Big Daddy, who reveals that the New Founding Fathers of America, the political party who enacted the purge, sanctioned death squads like his own because they believed too few lower-class people are purged, as well as to enforce an unwritten rule: do not save others. The officer is saved by Warren Grass, who had been spared. Big Daddy is shot and killed, and his followers stand-off with Warren, Eva, and Cali. Eventually, 7:00 AM rolls around, and the ensuing sirens mark the end of the annual Purge, forcing the death squad into retreat and leaving Eva and Cali to rush the officer to the nearest hospital.
