The film starts with young Charlie Roan and her family, all bloodied, being tied up by assailant. The assailant goes to kill the family except Roan.
Eighteen years later, two days before Purge Night, Senator Charlie Roan (Elizabeth Mitchell) attends a presidential debate as part of her campaign for the Presidency intending to end the Purge, stating how it only serves to eliminate the poor, the homeless and other lower classes of society that make up of the majority of America's population. It was her family that was murdered in front of her that night, leaving her as the only survivor. She is rivaled by Minister Edwidge Owens (Kyle Secor), a New Founding Father set up as a candidate to maintain the status quo. The NFFA, headed by Caleb Warren (Raymond J. Barry), view Roan as a threat to their rule and plot to use the upcoming Purge to eliminate her from play.
The day before the Purge, the NFFA revokes the Purge rule that protects ranking 10 government officials, appearing to attempt to reconcile with the people. Convenience store and deli owner Joe Dixon (Mykelti Williamson), his assistant Marcos (Joseph Julian Soria), and EMT Laney Rucker (Betty Gabriel) confront a pair of teenaged shoplifters who attempted to steal a candy bar. No longer able to afford his insurance premiums for the Purge, Joe decides to stake out and guard his store. Roan decides to wait out the Purge from her unsecured home in order to secure the popular vote of the common people. Her head of security, former police sergeant Leo Barnes (Frank Grillo), orders her home locked down and surrounded by security, accompanied by Chief Couper (Ethan Phillips) and Eric Busmalis (Adam Cantor). Meanwhile, a group of South African tourists arriving at Dulles Airport are interviewed by a news anchor wherein their response is to join the Purge. News of airports across America flooding with more tourists traveling to the United States to witness or join the Purge make the press dub them as "Murder Tourists".
After the Purge commences, Joe and Marcos repel an attack by the teenaged shoplifters, injuring their leader, Kimmy (Brittany Mirabile). Laney and her partner Dawn (Liza Colón-Zayas) patrol the city in an ambulance, rendering medical care to the wounded. Roan and Barnes are betrayed by Couper and Busmalis, who signal to a Neo-Nazi paramilitary force led by Earl Danzinger (Terry Serpico) that attacks Roan's house. Barnes manages to get Roan to safety, but is wounded in the process. He detonates a bomb in the house, killing Couper, Busmalis, and a few troops. Roan and Barnes try to seek shelter elsewhere, but are ambushed and taken captive by a group of Russian Murder Tourists. Before they are executed, Joe and Marcos shoot the gang dead and rescue them. As they take shelter in Joe's store, Marcos witnesses the teenaged shoplifters returning in two groups, causing Joe to call Laney for backup. As the four prepare to defend themselves, Laney arrives and runs over Kimmy with her ambulance, guns down the other shoplifters before finishing off a heavily wounded Kimmy with a point-blank headshot. The group then leave the store for a safer hideout.
With everyone safe in the ambulance, the group is ambushed by a helicopter piloted by Danzinger and two members of his team. They seek refuge underneath a highway overpass wherein Barnes deduces they were found because the bullet in his chest is a tracker. He extracts the bullet from while they are confronted by a large group of Crips, who calm when Joe gives the gang's trademark whistle call, revealing that he was once a member of the Crips. In exchange for Laney helping the gang leader's "boy", the Crips trick the paramilitary forces by planting the bullet in another area, and wipe out Danzinger's ground team.
The group is led to a hideout beneath a hospital protected by anti-Purge rebels and Dante Bishop (Edwin Hodge) where volunteer doctors and nurses administer to wounded Purge victims. Joe, Marcos, and Laney decide to go back to the store, but spot several NFFA death squad trucks heading to the hideout. Meanwhile, Roan discovers the rebels are planning to assassinate Owens and tries to dissuade them, as she wants to win the election fairly. They are forced to flee as death squad forces arrive, and the pair meet up again with the ambulance. However, the ambulance is rammed by Danzinger, and Roan is seized.
Roan is delivered to Owens at a midnight Purge mass in an NFFA-captured Catholic cathedral while Barnes and the others give chase. They meet up with the rebel assassination team led by Bishop wherein they infiltrate the cathedral through a tunnel system. Meanwhile, at the cathedral, Owens has an NFFA assistant Harmon James (Christopher James Baker) stab a man as a cleansing ritual for his drug addiction, before inviting the members of the NFFA to execute Roan. As Warren prepares to slit her throat, Marcos assassinates him, signaling the rebels to invade the cathedral and causing the enti