March 21st, 2033 - Just less than 2.5 hours before the Annual Purge commences. The employees of a diner are nervously waiting to be dismissed by their boss. They briefly discuss what they plan to do on the evening of the Purge. One of the waitresses, Eva (Carmen Ejogo), goes home to her father Rico (John Beasley) and daughter Cali (Zoe Soul). Cali is watching a video online of a man named Carmelo Jones (Michael K. Williams), who seeks to rally followers against the New Founding Fathers and their heinous acts against the poverty stricken population. Rico hates the Purge and wants to hear nothing about it for the night. He leaves, and Cali asks Eva if her boss gave her a raise. She tells her daughter that it's not in the cards at the moment.
A man named Leo (Frank Grillo) is arming up before the commencement. He hears a knock on the door. It's a woman named Janice (Chad Morgan). She sees newspaper clippings on his wall of a man that was let go from trial, knowing why he plans to go out that night. Although she urges him not to go through with his plans, Leo sends her away.
A couple named Shane (Zach Gilford) and Liz (Kiele Sanchez) are out driving. There is obvious tension between them. After their trip to a store, Liz tells Shane they have to tell his sister about something. They are briefly spooked by a punk with face paint and a cross drawn on his forehead. He retreats and joins his other masked/face-painted punk friends. A man in a mask with "God" written on it waves at the couple. As they hit the road again, we learn that they are separating, and this is what Shane is hesitant to tell his sister. Suddenly, their car breaks down in the middle of the street, not long before the start of the Purge. They look under their car and see that the wires were cut. The punks ride up close to them, with the "God" punk stepping out with a blade. The couple knows they cut the wires and they make a run for it.
As the clock nears 6:00, Eva and Cali prepare dinner, while Shane and Liz try to hide from the punks, and any other Purgers. The sirens go off, signaling the start of the Purge. Leo goes out in his armored car with his guns in tow. A truck with psychos drives through the streets. They pass a couple armed with machetes, looking to "release the beast". From inside the truck, a man known as Big Daddy (Jack Conley) takes out a machine gun and kills the couple. Leo passes a man getting beaten to death, while not noticing a burning bus flying behind him moments later.
Cali goes to find Rico to have dinner. She finds a note that she shows to Eva, where Rico discloses that he is already dying, and is going to the home of a rich family for them to purge on him. In return, they supply money to Eva's account to provide support. As the women mourn their inevitable loss, they hear commotion outside. They look out the window to see a SWAT-like team of men coming toward their building. The ladies then hear a loud banging noise. Their lecherous neighbor Diego (Noel Guliemi) comes in with a shotgun, planning to rape and kill the women. He grabs Eva's throat and licks her face, when another banging is heard. Diego is shot to death by the armed men, who then take Eva and Cali to be purged by Big Daddy.
Leo drives by the ladies' building to see the men trying to force them into their truck. Leo mutters to himself just to drive by and ignore this, but, seeing the ladies fight back, he chooses to get out of the car and shoot the men down. With his back turned, Shane and Liz sneak into his car to hide. Big Daddy emerges from inside the truck, and Leo shoots him in the face. He turns to walk back to his car, but then offers Eva and Cali to join him. They follow and are startled to see Shane and Liz in the back of the car. Leo draws his gun on them, but they assure him they're unarmed and are not looking to purge. They then see Big Daddy standing to his feet, and the punks on their bikes not far behind. Reluctantly, Leo takes everybody with him as Big Daddy fires up his machine gun.
Leo's car also happens to break down, leaving everybody stranded in the outskirts of town. Eva offers to get Leo a new car from her friend and co-worker Tanya (Justina Machado) if he promises to get them to her place safely. He agrees and hands everyone guns to protect themselves.
On their trek to Tanya's home, Shane gets his foot caught in a trap. Purgers from every corner try to shoot at the group, but Leo manages to fire back and kill most of the attackers. Shane shoots the cable and breaks himself free. As a running group of Purgers chase after the five, they run down by a tunnel. While walking, Cali tries to talk to Leo and convince him that whatever reason he had for being out at night is wrong. He just ignores her. Another group of Purgers rides in, wielding a flamethrower and more guns. The five run again while the Purgers kill other bystanders that were hiding. Shane and Liz shoot back at the Purgers while Leo, Eva, and Cali run. Shane gets shot in the shoulder, but he and Cali manage to shoot the driver and then hit the flamethrower's tank, blowing all the Purgers up.
The group finally makes it to Tanya's house. She and her mother, along with her sister Lorraine (Roberta Valderrama), set up dinner for the group. They are joined by Tanya's father and brother-in-law Roddy (Niko Nicotera). The ladies treat Shane's bullet wound, while Leo prepares to take the car and head out. Eva admits that there isn't a car and that she needed Leo to get them there safely. He decides that one way or another, he is going to get to his destination. He also doesn't trust Tanya, as she is clearly drunk and has been making comments about purging should she choose to. Moments later, Lorraine comes in with a gun and shoots Tanya to death, after finding out that she and Roddy were having an affair. Lorraine draws her gun on Roddy, but he grabs Liz and holds her at gunpoint to call Lorraine's bluff. Leo shoots Roddy in the side and then shoots Lorraine. The five run out (AGAIN) and try to escape. Unfortunately, Big Daddy and the armed men are outside waiting to get Leo, Eva, and Cali. The group tries to find another way out, but once they get outside, they are captured by the biker punks.
The punks take the group to a facility where they are paid to bring the people in as victims for an organized Purge. The five are brought before an audience of rich people that gleefully bid on killing them. After a group of seven Purgers are selected, the five are put in a dark room for the Purgers to hunt them down. As the Purgers say their thankful "prayer" of sorts, Leo devises a strategy to get himself and the others out of there. He sneaks by the Purgers and takes down one man, killing him and taking his night vision goggles and weapon. He kills three more and takes their weapons, to the horror of the rich audience watching. Leo shoots at them to frighten them, but the window protects them. He brings the weapons to the group, where they kill one more Purger before the other two escape. The elderly rich woman organizing the event calls in for back-up. The group, knowing they may not make it out, sits in fear. Shane apologizes to Liz for everything, and she tells him she loves him, just seconds before the back-up Purgers shoot Shane to death. Liz cradles his body in her arms and cries. Then, an explosion is heard. Since explosives are prohibited here, the Purgers sense trouble. The back-up Purgers are shot to death by the stranger from the first film that the Sandin family helped. He brings in an army, led by Carmelo. They fire at the other Purgers, and then offer to help out the group. They give Leo, Eva, and Cali a chance to escape, but Liz chooses to stay behind and purge to avenge Shane. Carmelo and his men vow to guard Liz. Leo finds the woman that organized the event and carjacks her at gunpoint, where she begs Leo not to shoot her. He lets her leave.
The morning comes, and the Purge is nearly over. Leo drives to the home of a man named Warren Grass (Brandon Keener), who happened to kill Leo's son during the previous Purge in a drunk driving incident. He was let go on a technicality despite his blood-alcohol level being very high. Cali pleads with Leo not to go through with this, but he ignores her and breaks into the man's home, just five minutes before the Purge is over. He goes into Warren's room and wakes him and his wife up. He holds Warren at knifepoint and reminds him of what he did to his son. Warren apologizes and acknowledges his wrongdoing, and then a scream is heard...
Leo walks out of the home, and is then shot twice. The shooter is Big Daddy, who looms over Leo, stating that he shouldn't have saved Eva and Cali, as they were his to kill. He adds that The Purge is no time or place to be a hero. Before he kills Leo, Big Daddy is shot in the head by Warren, whom Leo chose not to kill after all. The armed men led by Big Daddy come running up to the house, while Eva and Cali run out with their guns. And then the sirens go off, signaling the end of the Purge. Eva then urges Warren to get his car so that they can take Leo to the hospital.
On the ride to the hospital, Cali thanks Leo for saving them, also happy that he didn't kill Warren. They make it to the hospital, with Eva and Cali carrying Leo inside, while the helicopters fly over the city, slightly devastated by the events of the evening.
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