The story begins in an alternate 2014 where in a time of economic and social turmoil in the United States, combined with rising unemployment and a housing crisis leads to the foundation of a new ruling government called the New Founding Fathers of America, or the N.F.F.A. for short, led by their president Bracken, to replace the Democrats and Republicans as the most powerful political party in the country. An insane drug addict named Skeletor is interviewed by an employee of the N.F.F.A., who tells the man about his inner thoughts and how he plans to "purge" and unleash his hatred onto other people. The employee assures Skeletor that he will have a chance to do that very soon.
In 2017, the N.F.F.A.'s Chief of Staff, Arlo Sabian, and Dr. May Updale, announce that they plan to conduct an experiment in two days on Staten Island, where for 12 hours, citizens will be able to purge and release their inhibitions in various ways, murder included. The N.F.F.A. offers those who choose to stay $5,000, and additional compensation for those who choose to participate and survive. The participants are also outfitted with contact lens cameras so their activity will be monitored and tracking devices in case they leave the island.
Dmitri, a low-lying drug lord and businessman who lives on the island, tells his dealers that they will not leave, since moving his money and products would draw attention, and tells them to stay at a safe house to lay low. Capital A, another dealer, objects to this, choosing to defy Dmitri's orders and purge, while Isaiah, a newbie dealer, is attacked by Skeletor. Following the attack, Isaiah goes to see Nya, his sister as well as an anti-Purge activist and Dmitri's ex-girlfriend, for treatment. Nya also joins her friends Dolores, Luisa, and Selina to wait out. Dmitri, having chosen to stay at the island, is visited by two prostitutes named Anna and Elsa to keep him company. Skeletor commits the first Purge murder, and a video showing the murder goes viral by the N.F.F.A. They also observe that most of the crimes taking place on the island are minor ones such as looting, vandalism, disorderly conduct, and public disturbances, as opposed to the more violent ones like murder.
To obtain revenge on Skeletor, Isaiah secretly joins the Purge and eventually confronts Skeletor, but cannot bring himself to shoot him in cold blood. Isaiah runs off and encounters several other purgers, before going into hiding and calling Nya for help. Anna and Elsa are also revealed to be Purgers and attack Dmitri, but Dmitri manages to fight them both off and finds out that Capital A was responsible for sending them in so as to take over Dmitri's business for himself. Meanwhile, Skeletor finds Nya in the streets and captures her before attempting to rape her, but Isaiah wounds him before escaping with Nya. Capital A and his men then go to Dmitri's office, but upon their arrival, they are ambushed and killed by Dmitri's gang and Dmitri personally kills Capital A for his treachery. Dmitri spares Anna and Elsa, but not before telling them never to return to the island. Nya and Isaiah return to the church, but find blood soaked white supremacist purgers exiting the church. Once they enter the church, they find Luisa and Selina are safe, but Dolores is missing. However, once they arrive at Nya's apartment, they also find Dolores alive and well.
At the N.F.F.A. headquarters, Updale observes the recent footage of the events and finds out about the sudden increase in murder as well as masked participants entering the area. She soon learns that masked participants are mercenaries hired by the N.F.F.A. to kill civilians. Sabian explains that he sent the mercenaries in order to make the experiment look successful, as well as set the balance between the rich and the poor. Updale soon realizes the N.F.F.A.'s true intentions are to kill off the poor at the expense of social programs. Sabian then has Updale taken to a shady neighborhood, where she is executed by the N.F.F.A. mercenaries. Watching footage of her death, Sabian orders it to be deleted.
As Dmitri and his gang escape to the streets, they are attacked and Dmitri kills the assilants. He soon learns that his attackers were mercenaries hired by the N.F.F.A. and had also dumped weapons to provoke participation in the neighborhood. Dmitri then decides to take a stand and protect the neighborhood. He saves Freddy, a local shop owner and a friend of his as well as his associates, before proceeding to Nya's apartment to save her and her friends. However, Dmitri's gang is attacked by N.F.F.A. drones, killing most of them. Dmitri then calls Nya, warning her of the mercenaries and telling her he will come for her, before entering the building. He kills most of the mercenaries as they gun down several of the residents, eventually reaching Nya and her group and helping them hide in a safe place. However, they are soon attacked and cornered by a second wave of mercenaries who are armed with a rocket-propelled grenade when Skeletor suddenly appears and kills several of the mercenaries before he is eventually taken out himself. Dmitri then tosses a plastic explosive, and despite being wounded, manages to shoot it, causing a massive explosion that wipes out the remaining mercenaries and their leader. The sirens then signal the end of the Purge as Dmitri and the group leave the building and are hailed as heroes, and Dmitri encourages the others to fight back.
During the end credits, Sabian issues a statement announcing that the experiment was a success, and next year, a nationwide Purge may soon follow, leading to the events of the other films as well as the TV series.
