A crazed drug addict named Skeletor discusses his dark thoughts, including his desire to “purge” and unleash his hatred on other people. An NFFA employee tells Skeletor that he can very soon.
In an alternate 2014, the New Founding Fathers of America (NFFA) are now the most powerful party in the United States. In 2017, NFFA chief of staff Arlo Sabian and sociologist Dr. May Updale announce an experiment to take place on Staten Island where for 12 hours, citizens will be allowed to purge and release their inhibitions in any way they choose, including murder. The NFFA offers residents $5,000 to stay during the experiment, with additional compensation if they join the purge and survive. They also outfit the participants with contact lens cameras so that they can monitor all activity and put tracking devices in them so they will know if they try to leave the island.
Drug lord Dmitri tells his dealers that they will not be leaving the island as it will draw too much attention, moving his large amounts of money and product. Dmitri says to his soldiers that they will stay in a safe house and lay low. Dealer Capital A decides to defy this order and go out and purge, while newbie dealer Isaiah is attacked and cut by Skeletor. Isaiah goes to his sister Nya, an anti-Purge activist and Dimitri’s ex-girlfriend, for treatment.
As people flee Staten Island, Nya joins her friends Dolores, Luisa, and Selina in a church to wait out the Purge. Dmitri has stayed behind because Capital A has sent Anna and Elsa, two prostitutes, to his office. Skeletor commits the first-ever Purge murder, and the video recorded by the NFFA goes viral. The NFFA also observe that more parties than killings are taking place and that crimes are mostly just looting and vandalism, as opposed to the expected murder.
Isaiah secretly joins the Purge to get his revenge on Skeletor. He eventually confronts him during the Purge but cannot shoot him. Isaiah runs off and into more purgers, before hiding and calling Nya for help. Anna and Elsa are revealed to be Purgers who attempt to kill Dmitri, but Dmitri fights them off and learns that Capital A had sent them in an attempt to take over Dmitri’s business. Skeletor captures Nya in the streets and attempts to rape her, but Isaiah wounds him and they escape.
Nya and Isaiah return to the church to see blood-soaked white supremacist bikers leaving it. Although Luisa and Selina survived, Dolores’ fate is unknown. They all return to Nya’s apartment where Dolores has managed to make it safely. Meanwhile, at the NFFA headquarters, Updale becomes suspicious of a sudden increase in murder, along with the presence of masked participants. She reviews the video footage of the Purge and notices vans full of masked killers arriving. Much to her horror, she finds out that the said masked killers are trained mercenary groups killing multiple civilians. Sabian explains that he sent the mercenaries to make the experiment look successful, and eventually to help balance the wealth disequilibrium amongst the rich and the poor. Updale protests this tampering, realizing that the NFFA only wants to eradicate the poor to save the expense of social programs. Knowing that his corruption has been exposed, Sabian has Updale taken away and killed, and erases all footage of the event.
Capital A and his crew go to Dmitri’s office to thank Anna and Elsa, but Dmitri and his gang kill them all. Dmitri and his gang escape through the streets, until unknown assailants attack them. Dmitri kills them all and discovers they are mercenaries. Realizing they were sent by the NFFA, who had also dumped weapons into the neighborhood to provoke participation, Dmitri and his crew decide to take a stand against them and protect the neighborhood.
After saving local shop owner and friend Freddy and his associates, they go to Nya’s apartment building to try and save her and her friends, but NFFA drones shoot and kill most of Dmitri’s gang. Dmitri calls Nya and warns her of the mercenaries’ invasion, but he is coming to help her. Dmitri successfully kills a number of mercenaries and helped Nya’s group to hide in a safe space. The mercenaries are about to shoot a rocket-propelled grenade into the apartments when a deranged Skeletor arrives and kills some mercenaries before he himself is killed. Dmitri grabs a piece of plastic explosive, throws it and shoots it repeatedly until it explodes, killing the remaining mercenaries. As sirens sound the end of the Purge, a wounded Dmitri is hailed as a hero, and he states that the survivors must somehow fight back.
During the end credits, Sabian reads a statement calling this experimental Purge a success, and that a nationwide Purge may begin as soon as the following year. A mid-credits scene leads into the events of the other films, and directly ties into The Purge television series.