The film starts on Purge Night. A couple of masked men are keeping a family tied up. One of the men says they are going to play "Mommy's Choice" to see which one of them will live through the night.
18 years later. Two days before Purge Night.
We see a news report on Senator Charlie Roan (Elizabeth Mitchell), who is running for presidency and is aiming to get rid of 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. Purging has become so popular that foreigners are visiting the U.S. to get in on the action. Running against her is Minister Edwige Owens (Kyle Secor). In a secret meeting in Washington DC, the New Founding Fathers of America counsel want to get rid of Charlie by any means necessary. The leader and chairman, Caleb Warrens (Raymond J. Barry), has something sinister in mind.
Meanwhile we are introduced to Joe Dixon (Mykelti Williamson), a convenience shop owner. He runs the place with his assistant Marcos (Joseph Julian Soria), while his friend, an EMT named Laney (Betty Gabriel), pays them a visit. Marcos spots two uniformed schoolgirls attempting to shoplift some candy. Joe confronts them, leading one of the girls, Kimmy (Brittany Mirabile), to rudely dismiss him. She is forced to do so when Laney steps in, as Kimmy idolized Laney when she was known as "Pequeña Muerte" (Little Death) in school. Joe then gets a call that his Purge insurance rates have been jacked up, and if he doesn't pay by the next day, his store won't be protected. He decides to guard the store himself.
Charlie is guarded by Leo Barnes (Frank Grillo), who also wants to put an end to Purge Night. It has been two years since Leo attempted to kill the person who killed his son on the last Purge Night (from the previous film). Charlie decides to stay in her own home instead of a bunker so that she can stay on the good side of voters.
Purge Night arrives. The announcement goes off that no one, including elected officials, is off limits tonight (in complete contrast to certain high-ranking Level 10 officials in the previous two Purge films). Leo secures Charlie's home, with snipers standing watch outside. A few minutes later after the commencement, all the snipers are shot dead by unseen marksmen. Leo finds out that Chief Couper (Ethan Phillips) and Tall Eric Busmalis (Adam Cantor) have betrayed them, and are allowing an armed mercenary team led by a skinhead named Earl Danzinger (Terry Serpico), to storm the house. Leo guides Charlie out of the house through a tunnel beneath the house. He then sets off a bomb he left, killing the villains left inside the house including Couper and Tall Eric.
While Laney patrols the city in her van, Joe and Marcos sit on the roof of the store to keep an eye out for Purgers. A car covered in Christmas lights rolls up. Three girls with creepy masks step out. One of them takes off her mask, revealing her to be Kimmy, her face caked in blood. She claims to have just killed her parents, and she plans to break into the store to retaliate against Joe. Marcos scares her off by shooting her which the bullet only gazes her ear.
Charlie and Leo run out into the streets, trying to avoid the Purgers. They are ambushed by a small group of foreign Purgers. As one of them brags about how they'll kill them, Joe and Marcos show up and shoot all the Purgers. They take Charlie and Leo inside the store for protection. Joe calls Laney to tell her that they have Charlie with them.
Kimmy and her goons return to break into the store. They try to cut through the place with power saws, but Laney shows up and plows Kimmy down with her van. She steps out and shoots the other girls before blowing Kimmy's brains out. Laney urges everyone to hop in the van. Another shoplifter is trying to break in from the back. Joe wants to stay, but Laney won't let him, and when Marcos says he'll stay if Joe stays, Joe decides to join them.
The group is shot at by a helicopter. Leo pulls a bullet from his chest from the raid at Charlie's house when he discovers a tracker bullet that led the villains to Charlie. As they continue moving, they are followed by some Crips gang members also out tonight on Purge Night. Joe manages to get them to back off by doing the Crips whistle, revealing his own past as a gang member. The Crips ask the group to help one of their own in exchange for a favor. When two of Danzinger's teammates show up and find the bullet, they run into the Crips, who shoot them dead.
The group goes to a hidden location led by Dante Bishop (Edwin Hodge), the new leader of the anti-Purge movement. Many victims are being treated there by professionals. Charlie tries to persuade Dante to not kill Owens. A death squad infiltrates the place, forcing the group to run. Their van gets rammed by Purgers, and Charlie gets taken.
Charlie is brought to Owens' church for a Purge mass. Warren prepares to slit her throat, but the others arrive in time, and Marcos kills Warren. The rest of the congregation flees. Dante considers killing Owens, but Charlie and Leo talk him out.
Danzinger and his men show up and start Purging. He kills Dante and fights with Leo, ending in Leo killing him. Owens's assistant comes out with a gun and heads toward Charlie. Joe steps in and shoots at the man, taking a few shots himself before getting the guy with a headshot. Joe succumbs to his wound and states his hope for Charlie to win and for Marcos to take care of the store.
Two months later. Election Day.
Charlie wins the presidency in a landslide and proceeds to start the plan to get rid of Purge Night. Marcos and Laney now take care of Joe's store in his memory. Before the end credits begin, we hear a news report that Purge supporters are reacting to the election results with violence.