In 2014, the New Founding Fathers of America, a totalitarian political party portraying themselves as successors to the Founding Fathers of the United States, are voted into office following an economic collapse and pass the 28th Amendment which sanctions an annual national civic tradition called "The Purge" the first of which takes place in 2017. The Purge occurs for 12 hours, from 7 p.m. March 21 to 7 a.m. March 22, during which all crime is legal and all police, fire, and medical emergency services remain unavailable. Restrictions prohibit government officials "ranking 10" from being disturbed, as well as the use of all weapons above Class 4 (explosive devices such as grenades, rocket launchers, and bazookas). Violation of Purge rules results in a summary execution by hanging. The Purge has resulted in unemployment rates plummeting to 1%, low crime, and a strong economy.
On March 21, 2022, James Sandin, a top salesman for elaborate security systems designed specifically for Purge Night, returns to his home in an affluent Los Angeles gated community to wait out the night with his wife, Mary Sandin, and their two children, Zoey Sandin and Charlie Sandin. The family is assured that the security system manufactured by James company will keep them safe. Their neighbors attribute the size and fittings of the newly extended Sandins' house to his success in selling security products to them for Purge Night. While the family awaits the start of the Purge, Zoey meets her boyfriend Henry, an older boy whom James dislikes.
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 managed to sneak back in before the security system was engaged, and says that he plans to confront her dad 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 as Henry comes downstairs and pulls a gun on James. Henry fires at James and misses, but James fires back, mortally wounding and eventually killing Henry. During the chaos, the wounded man disappears and hides. James takes Mary and Charlie back to the security control room.
As James reprimands Charlie for letting the man into their home, they view over the surveillance cameras, where they witness a gang of masked young adults armed with guns, axes, and hammers arriving in the front lawn. Their leader unmasks himself, compliments the Sandins on their support of the Purge, and then tells them that if they fail to surrender the man, they will be forced to "release the beast," implying they will forcefully enter the house and kill everyone inside. Mary asks James if the security system will help protect them, but James admits the system is essentially a security theater—it is only supposed to discourage potential invaders and would not actually protect them against heavy force. They decide to find the man and give him to the Purge gang outside, but after capturing the man, they realize they are no better than the gang waiting outside. They decide to spare the him and defend themselves against the gang.
With their deadline having passed, the gang uses a truck to rip the metal plating off the front door and enter the house. As the family fights against the gang, Charlie views the surveillance cameras and notices their neighbors leaving their homes. The neighbors overpower and murder the gang, though James is mortally wounded by the gang leader in the battle. Elsewhere, Mary is subdued by two Purgers, one of whom tickles her before almost killing her, but are both killed by the neighbors. As the gang leader prepares to kill the remaining Sandins, Zoey appears and kills him.
Mary thanks their neighbors for their support, but one of them, Grace Ferrin, reveals their hatred for the Sandins due to their wealth acquired by the money the neighbors paid the Sandins with for various security products. They tie Mary, Charlie, and Zoey up with duct tape, pulling them out into the hallway to kill them. But as the neighbors make final preparations for the murder, the man re-appears, kills a neighbor with a Browning Hi-Power 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 blares, 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.
News reports later state that this year's Purge is 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.
- Ethan Hawke as James Sandin
- Lena Heady as Mary Sandin
- Max Burkholder as Charlie Sandin
- Adelaide Kane as Zoe Sandin
- Edwin Hodge as Dante Bishop
- Tony Oller as Henry
- Rhys Wakefield as Polite Leader
- Arija Bareikis as Grace Ferrin
- Dana Bunch as Mr. Ferrin
- Chris Mulkey as Mr. Halverson
- Tisha French as Mrs. Halverson
- Tom Yi as Mr. Cali
- John Weselcouch as Freak Interrupting
- Alicia Vela-Bailey as Female Freak
- Peter Gvozdas as Dr. Peter Buynak