In the year 2022, Charlie Roan was a young woman who wound up being abducted, along with her entire family, during the annual Purge. She survived, but the rest of her family perished that night.
Her experience with the Purge inspires her to enter the political arena in order to see the annual holiday abolished. In the eighteen years that followed, she became a Senator and a front runner for President of the United States, her campaign being based on her ridding the nation of the Purge, which she states is actually a method of population control that targets the impoverished and homeless of the nation. With her popularity rising, Caleb Warrens, the leader of the incumbent New Founding Fathers of America, sees her as a threat to their reign and plans to use the Purge, which commences in two days, to eliminate her. To that end, he revokes the rule protecting government officials with a clearance level of 10 or higher, making all government officials, including Roan, fair game.
The day before the Purge, Joe Dixon, the owner of a deli and bodega in Washington DC, confront a pair of teenaged girls trying to shoplift, along with his assistant, a Mexican immigrant named Marcos, and a friend, EMT Laney Rucker. Joe then receives news that his Purge insurance premiums have been increased. No longer able to afford the insurance, and against the advice of his friends, he decides to stay at the store that night to protect it himself.
Meanwhile, Roan, also against the advice of her head of security Leo Barnes (the former police officer from The Purge: Anarchy), decides to wait out the Purge at her private residence, afraid that hiding out in a shelter in a concealed location could cost her her political momentum. Barnes, unable to convince her otherwise, makes preparations to ensure her home is secure.
On March 21, 2040, at 7:00 PM, the annual Purge commences. With the aid of Marcos, Joe repels an attempt to break into his store by a gang lead by one of the girls who tried to shoplift from him the previous day. All the while, Laney patrols the streets in an armored ambulance with her partner, Dawn, tending to those who had been injured during the Purge.
Meanwhile, traitors in Roan's security detail allow a group of white supremacist mercenaries, lead by neo-Nazi Earl Danzinger, entrance into the senator's house. Barnes, realizing something is amiss, leads Roan out of the house, setting up a bomb in her room to take out some of the mercs, but getting shot in the escape. As they try to find shelter, they are accosted by a group of Russians who came to America to participate in the Purge as "murder tourists". Luckily, they were right close to Joe's store, and are rescued by Joe and Marcos, who shoot the tourists.
Roan and Barnes take shelter in the bodega. Not long after, however, the teenaged shoplifters return with reinforcements, determined to break in. Joe calls Laney for help, and she eventually comes just as the shoplifters are about to breach the store, shooting them with a shotgun. She was delayed by a patient she had found, a young man named Rondo, who desperately wanted to purge and had to be cuffed down.
The group escape in Laney's ambulance, but are pursued by the skinhead mercs, who attack them from an attack chopper. Barnes realizes the bullet he took earlier was a tracer bullet, and as the group take shelter under an overpass, extracts the bullet himself. Unfortunately, they are not safe, as the ambulance is surrounded by a crowd of rowdy gangsters. Joe then gives off a familiar whistle, revealing that he was once a member of one of the most notorious gangs in America, the Crips. The Crips settle down, and one of them asks for help in the treating leader's wounded "boy". They agree to help in exchange for fighting back against the skinheads, allowing them to make their getaway.
Laney drives the group to an underground shelter run by Dante Bishop and anti-Purge insurgents, where they treat those who had been injured during the Purge. As Joe decides to go back to his store to defend it with a reluctant Laney and Marcos in tow, Roan and Barnes uncover a plot by the insurgents to assassinate Caleb Warren and other higher-ups in the NFAA during a "Purge Mass" being held at Our Lady of Sorrow Church. This outrages Roan, as she believes killing Warren and others would make him a martyr, losing her support for election: she wants to win the election fairly. Worse still, Joe and his companions find death squads en route to the insurgent hideout. They return to rescue Roan and Barnes, but are intercepted by the skinheads, and Roan is abducted.
Roan is taken to the "Purge Mass", where Minister Edwidge Owens, her political rival, leads the mass, starting the evening by having Harmon James, an NFFA assistant, stab a drug addict to death in order to "cleanse" him.
Barnes and company give chase, meeting up with Bishop and his insurgents. They make it to the mass as Caleb Warrens is preparing to slit Roan's throat. The insurgents open fire on the mass, with Marcos shooting Warrens in the head and causing whoever isn't killed in the ensuing gun fight to flee. Bishop captures Owens and contemplates killing him, while Roan pleads with him to spare him, telling him that he would be no different than the men he wanted to see deposed. Eventually, she appeals to Bishop's better nature: even as Owens tries to goad Bishop into killing him, Bishop lets him live, telling Roan that she had better win.
As the group find several bound and gagged victims, they are attacked again by the skinheads. Most of the insurgents are killed, but a wounded Bishop manages to dispatch them while Barnes kills Danzinger in single combat. As the bound victims are freed, one runs to the exit, but before he can open the door, Harmon James appears with shotgun in hand, killing the victim, wounding Marcos, and incapacitating Laney. Joe intervenes, killing Harmon James, but is mortally wounded in the process. Before he dies, he urges Roan to win, and asks Marcos and Laney to watch after his store.
In the months that followed, Roan's popularity continued to rise, eventually allowing her to win the election by a landslide, with her first order of business being an executive action abolishing the Purge. As Laney and Marcos renevate Ben's store, news breaks of angry NFFA supporters protesting Roan's presidency, inciting violent riots.
