In 2014, the New Founding Fathers of America, a totalitarian political party, are voted into office following an economic collapse. They pass a law that sanctions an annual “Purge”: for 12 hours each year all crime, including murder, arson, theft and rape, is legal during the period, except against government officials, and all emergency services are unavailable until 7 am.
By 2022, the United States has become virtually crime-free and the unemployment rate has dropped to 1%, ostensibly because of the Purge.
James Sandin returns to his home in an affluent Los Angeles gated community to wait out the night with his wife Mary and their children, Zoey and Charlie. The family is assured that the security system manufactured by James’ company will keep them safe. While the family awaits the start of the purge, Zoey meets her boyfriend Henry, an older boy whom James dislikes as he is eighteen, deeming him too old for his daughter. James enables the security system, and as the purge begins, the family disperses in their home to go about their normal routines.
Zoey returns to her room to unexpectedly find Henry, who had snuck back in before the security system was engaged, and says that he plans to confront her father about their relationship. Meanwhile, Charlie watches the security monitors, and sees a wounded man calling for help. He temporarily disables the system to allow the man into the house. James races to re-engage the system and holds the man at gunpoint. Henry comes downstairs and pulls a gun on James, but James fires back, mortally wounding Henry. During the chaos, the wounded man disappears and hides.
Through the surveillance cameras, the family witness a gang of masked, heavily armed people arriving at the front lawn. The leader warns them that failure to surrender the wounded man will result in invasion. Mary asks James if the security system will protect them, but James admits the system will not actually resist a determined assault. They decide to find the man and give him to the purge gang outside, but after capturing him they realize they are no better than the gang. They decide to spare the man, and defend themselves against the gang.
With their deadline having passed, the gang uses chains attached to a truck to rip the metal plating off the front door and enter the house. James fights back with a shotgun and kills several gang members before he is stabbed by the leader. Charlie views the surveillance cameras, and notices their neighbors leaving their homes. The neighbors overpower and kill the remaining gang members. Mary is captured and humiliated by two purgers, one of whom tickles her. Before they can finish her off, they are killed by the neighbors. The gang leader reappears, brandishing a shotgun to kill the family, but is fatally shot by Zoey, who is wielding Henry’s handgun. James succumbs to his wound and dies in front of Mary, Zoey, and Charlie, leaving them in tears.
Mary thanks their neighbors for their support, but one of them, Grace Ferrin, reveals their hatred for the Sandins due to the wealth acquired at their expense. They tie Mary, Charlie, and Zoey up with duct tape, pulling them out into the hallway to kill them, but as the neighbors make a ritualistic preparation for the murder, the man whom Charlie let in earlier re-emerges. He shoots Mr. Cali, one of the neighbors, dead and holds Grace hostage, forcing the neighbors to free the Sandins. Mary prevents the stranger from killing the other neighbors, deciding that they will all wait out the remainder of the Purge without any violence.
The next morning, in the last few minutes of the Purge, Mary, Grace, the stranger, and the neighbors wait in the living room. Mary asks a neighbor, Mr. Halverson, if he enjoyed Grace’s “Purge party” (the meeting of the neighbors to discuss the plans for the murders), to which Halverson affirms. As Charlie and Zoey mourn James’ death, Grace attempts to seize a shotgun from Mary; Mary regains control of the shotgun, rams the stock of the gun into Grace’s nose and throws her head down onto a glass table, furiously ordering for an end to the violence. Mary orders Grace and the neighbors out of the house as the siren sounds, signaling the conclusion of the Purge. After the neighbors depart, Mary thanks the stranger for his help and he bids the Sandins good luck as he leaves the gun behind that he was using.
During the credits, news reports are heard, which state that this year’s purge was the most successful to date. Other stations broadcast that the stock market is booming due to the massive sales of weapons and security systems. A man’s voice speaks of the loss of his patriotism after the death of his sons the night before.