It’s 2022. Crime in the USA is at an all-time low of 1% thanks to an annual Purge that takes place over 12 hours from 7pm on 21st March. During the Purge, people can commit any crime they wish without fear of consequence against anyone except Category 10 US government officials. The emergency services are not available, so there's no ambulance, police or fire service for those targeted. Critics of the Purge say that it’s used by the rich to target and punish the poor as they’re the only people who can’t defend themselves.
James Sandin (Ethan Hawke) sells home security systems that lock down people’s houses, keeping them safe during the Purge. He’s very successful and has sold security systems to all of his neighbours. As he goes home to get ready for the Purge to start, he’s told that he’s the number one sales person for his company. He stops just before his house to give some advice to a neighbour on maintaining a security system that he's bought.
Mary (Lena Headey) is getting dinner ready. A low level camera creeps up on her, startling her. The camera’s inside a half-burnt doll and mounted on a radio controlled truck that has microphones attached to it. Mary talks to the truck, telling her son Charlie to come and help her get dinner ready. Charlie moves out of a secret hiding hole he’s created in his closet (which is wallpapered with his violent drawings of the Purge) and explains that he’s put insulation around the truck so that there’s no noise. Mary congratulates him and tells him to set the table.
Upstairs, daughter Zoe is snogging her boyfriend Henry. Henry is 18 and James has forbidden him from seeing Zoe because he’s too old for her. Henry wants to speak to James, but Zoe tells him not to as it would only anger him more. She tells Henry that he’s got to leave so that he can get home before the Purge starts. He climbs down from her bedroom window and disappears across the front lawn.
Mary goes outside to leave a bunch of blue flowers outside the front gate. People are encouraged to do this during the Purge to show that they support it. She’s met by her neighbour Grace who wants to give her a plate of cookies. Grace comments on how beautiful Mary’s house is and the big extension that they put onto it as a result of the bonus that James got the previous year. Mary asks her if she’s having a Purge party this year and Grace says that she isn’t she and her husband are just planning to stay in behind the security system that James sold them. A couple more neighbours drive by and stop to say hello to Mary and Grace.
The scene cuts to the family having dinner. James asks the family how their day went and invites Zoe to start. Zoe is clearly annoyed with her dad over the Henry situation and asks if she has to go through this pretense. James waits silently until she gives in and tells him a bit about school. James goes next and tells the family about his being the number 1 salesmen and how this will mean a big bonus that year. The family congratulate him. Charlie then goes next and says that for English class he wrote a story about a man who loved so much that he could actually kill people with it. Zoe makes a joke that Charlie’s story would be better if he’d written it about a penis and the family laugh and start to relax.
After dinner they go into a room with TV screens showing the view from different CCTV cameras based around their property. Mary notices that a group of people are heading over to Grace’s house and says that they must be having a Purge party after all. She’s evidently irritated by this. James notices that 2 more neighbours are heading out with automatic weapons and says that he didn’t realise they liked to hunt during the Purge. Charlie asks his parents why they don’t go out to hunt (implying that his parents are unusual by not doing so). James tell him, and he and Mary have never felt the need to because they don’t need to unleash the violent feelings that the Purge is designed to release. He says that notwithstanding this, both he and Mary completely support the Purge and believe that it’s necessary for the US and has turned society around into a more peaceful and prosperous one. Mary looks very uncertain during this speech but doesn’t contradict him.
James turns on the security system, using a number code that everyone else can see. Arming the system brings a series of shutters and bars down over all of the doors and windows. James then opens up a gun cupboard (which has a number of weapons in it) selecting a handgun, which he fills with bullets and keeps with him “just in case”. Zoe goes back to her room, James goes to finish up some paperwork in his office, Mary works out on a treadmill and Charlie goes back to his hidey hole, letting his camera-car run around the house spying on everyone.
Zoe is listening to music on her ear buds when she thinks she hears a noise. She takes out the buds but can’t hear anything else. She goes to her en suite bathroom. When she comes out she’s grabbed by someone and realises that it’s Henry. He tells her that he came back because he wants to talk to James man-to-man about letting him see Zoe. Zoe’s still not sure, saying again that she’s scared it’ll make things worse, but Henry runs through some of the arguments he’s preparing to use, including by emphasising how mature Zoe is for her age. This pleases her into letting him leave her room.
During all this Charlie’s driven his camera-car into the CCTV room where he’s noticed that one of the cameras has shown a man (the Bloody Stranger) staggering down the road outside their house. The Bloody Stranger has clearly been injured and looks homeless (he’s also wearing dog tags, implying he’s ex military). He’s shouting for help, begging anyone to take him into their house and give him somewhere safe to spend the night. Guns are fired a short distance away. No one else lets down their security systems. Charlie leaves his hidey hole and goes to the CCTV room. He sees that the Bloody Stranger is now outside their house, still shouting for help. After a moment’s hesitation, Charlie disarms the house’s security system and goes to the front door where he shouts for the Bloody Stranger to come into his house.
Everyone hears the barriers going up. James dashes into the security room and arms the security system again, sending the barriers down. He and Mary go into the hall where they see that the Bloody Stranger has managed to duck under the closing barrier and is now in the house. James pulls his gun and asks Charlie what he’s done. Charlie explains that the Bloody Stranger was hurt, and no one was helping him. James says that it isn’t his problem and that the Bloody Stranger has to leave because they don’t know who he is and he could be there to hurt them. At that moment, Henry calls to James from the stairs and Mary screams that he’s got a gun. James moves out of the way just as Henry fires. James also shoots, hitting Henry in the stomach. Zoe screams and runs to help Henry get back up the stairs. Mary goes to help James, who’s unhurt. In all the confusion, the Bloody Stranger disappears into the house.
Zoe drags Henry back into her bedroom. She can’t stem the blood and asks him why he did what he did. He gurgles that it was the only way they could be together. Henry dies of his wounds on the bedroom floor and Zoe, crying, leaves the room.
James, Mary and Charlie have gone into the CCTV room. James tells them to stay in the room and lock the door while he goes to see if Zoe is okay. Mary doesn’t offers to go instead as Henry didn’t try to shoot her. James refuses, concerned that the Bloody Stranger might hurt her. He goes to Zoe’s room where he finds Henry dead and takes a moment over his body. In the background, you see the Bloody Stranger in the hallway, who slips away before James can notice him. James goes back down to the CCTV room and tells them what's happened. He and Mary go out in the hall to argue about what to do next. While they’re arguing, Charlie is watching the TV cameras and sees that a group of people have turned up on their front lawn, all wearing masks and carrying weapons. He calls out to his parents that they need to see what’s happening.
They go back in and see a trio of people outside their front door. A young man steps up to the camera and lifts up his mask. He’s a peppy college type from a good family (the Polite Stranger) who says that he and his friends were doing their duty and trying to Purge by selecting the Bloody Stranger for their hunt, but the Bloody Stranger fought back and killed one of his friends. They followed him, and the Standins neighbours told them that they had offered the Bloody Stranger sanctuary in their house. It’s clear that he and his friends are rich snobs who kill the homeless and the poor because they see them as lowlife scum. Because of the blue flowers outside the front gate the Polite Stranger knows that the Standins are good people who support the Purge, so he’s giving them a chance to send the Bloody Stranger outside. If they don’t do, then he has more friends coming who will help him to get into their house and they'll kill the whole family.
His friends then cut the main power to the house, plunging it into darkness. Because the security cameras are on a separate generator, they’re the only devices that remain on.
The family argue over what to do next. James admits that the security system isn’t invulnerable, and it’s possible that the strangers could get in. Charlie doesn’t want to hand the Bloody Stranger over, but James insists that they have to because it’s the only way he can keep his family safe. He goes over to the gun cabinet and takes out another weapon, which he gives to Mary. She’s clearly uncomfortable with the plan but goes along with it. Charlie stays where he is.
There are then a number of creepy scenes following James and Mary as they go through the house looking for the Bloody Stranger while Charlie also looks for him using his camera-car. Charlie finds the Bloody Stranger hiding behind a sofa. By flashing his lights and driving the car up and down, he persuades the Bloody Stranger to follow him to his hidey-hole where the Bloody Stranger locks himself away. The scene cuts to the view outside, where the Strangers are becoming increasingly agitated, their behaviour towards the cameras becoming more threatening and violent.
Charlie is driving his camera-car to find out where his parents are when he suddenly finds Zoe in one of the hallways. She tells Charlie via the camera-car that she’ll be okay because she’s got Henry’s gun but she’s going to go to his hidey-hole to ride out the Purge and tells him to keep himself safe. Charlie’s clearly freaking out because he knows that the Bloody Stranger is there, and he dashes out of the CCTV room to warn her.
James and Charlie converge on the hallway outside Charlie’s hidey-hole where they find the Bloody Stranger is holding a knife against Zoe’s throat. He tells them that they can’t send him outside to be killed as he’s done nothing wrong. James tells him that he has to be a man and take what’s coming because otherwise they’ll kill the Standins. James sees that Mary is coming up behind the Bloody Stranger. He keeps the Bloody Stranger talking so that Mary can take the shot, but Charlie shouts out a warning. The Bloody Stranger whirls around, throwing Zoe hard into the wall where she hits her head and collapses onto the ground. Mary and Charlie rush to help her while James and the Bloody Stranger fight. James eventually knocks the Bloody Stranger out.
He and Mary take Zoe and the Bloody Stranger down to the CCTV room where they tie up the Bloody Stranger with duct tape. Mary is clearly very uncomfortable with what’s happening and is worried about Zoe. James keeps reassuring her that they’re doing the right thing, telling her that the plan is to tie the Bloody Stranger to a chair and drag him outside for the strangers to kill. He reminds Zoe that it’s the Bloody Stranger or the Standins. While they’re trying to tie him up, the Bloody Stranger wakes up and starts to struggle. James yells at Mary to pick up a letter opener and press it into a wound on the Bloody Stranger’s body to make him co-operate. Mary does so reluctantly and the Bloody Stranger screams. Zoe then wakes up because of the noise. Mary drops the knife saying that she can’t do this because it isn’t right. She goes to comfort her daughter, telling her that everything will be okay. Zoe’s seen what they’re doing to the Bloody Stranger and says that nothing will ever be okay again.
At that moment, the Polite Stranger comes up to the camera to say that time’s up. Because the Bloody Stranger isn’t outside, they’re going to tear down the defences and kill them all. The Bloody Stranger hears all this and tells James to take him outside. Instead, James takes out the rest of his guns and says that they’re going to defend their home. He tells Charlie to go and hide in the basement and gives weapons to his wife, telling her to defend the back of the house and shoot on sight anyone who tries to come in. Zoe goes to hide in her room. There's a final shot of the Bloody Stranger lying on the floor trying to free himself from the duct tape.
The Strangers use a truck to tear down the bars over the front door. At the same time, other Strangers are looking to break in at other points around the house. One Stranger comes through a window and goes to open the front door, allowing in the Polite Stranger.
The scene cuts down to the basement where Charlie is hiding in a corner. He hears noises outside and turns out his flashlight. When he thinks the coast is clear, he pokes his head out, only to find a Stranger. He’s dragged out of his hiding place and thrown about the room. James bursts in and shoots the stranger.
Mary is wandering around the back of the house. One of the Strangers is following her. Zoe is in her bedroom hiding under bed. She hears the Polite Stranger outside who’s found a picture of her on the wall and tells one of his friends that she’s exquisite, and he wants to save her until last (implying that he’s planning to rape her before killing her). A stranger comes into her room and up to the bed but doesn’t find her. The scene jumps back to Mary who’s still wandering around the house, followed by the other Stranger who eventually gets shot by James.
James goes to the games room and finds 3 Strangers. There’s a massive fight complete with beatings, choking, beating with pool cues. He beats 2 strangers unconscious and kills the third with a fire axe to the back. He finishes up by shooting each stranger in the head to make sure they’re dead but. As he leaves, the Polite Stranger steps out from behind a door and stabs him in the gut. The Polite Stranger thanks him for making this a very special Purge and leaves.
The scene then cuts back to the CCTV room where you see that the Standins' neighbours are coming up the drive into the house.
Mary finds that James has crawled out onto the stairs. She screams, drawing out Zoe and Charlie who all gather around him. Suddenly the Polite Stranger emerges with a gun. He gives a speech about how great the Purge is and how this is their duty as US citizens when he’s suddenly shot dead by one of the Standins’ neighbours. Mary’s crying and thanking them when Grace tells her to stop. They’re not here to save them. They here because they saw that the Strangers had come to kill them and realised that they wanted to kill the Standins themselves.
They take Mary, Zoe and Charlie into the front room. Mary’s still crying and asking why they’re doing this and Grace says it’s because the Standins are hated by all the neighbours. They’ve seen how James and Mary have got rich by selling them security systems and think they flaunted their wealth by building the extension. Mary pleads for them to spare Charlie and Zoe, but Grace tells her to shut up as they’re all going to die so Mary should show some dignity and stop scaring the kids. Grace says that she wants to go first with her knife and then the others can take it in turns.
Suddenly the Bloody Stranger shows up and shoots one of them. He holds the gun on the other neighbours and tells them to drop their weapons, which they do. He asks Mary whether she wants him to kill them. Mary says that she’s sick of killing. She says that she just wants to sit out the rest of the night in peace.
They all gather in the dining room, the neighbours sitting at the table while the Bloody Stranger holds a gun on them to make sure they don’t try anything. Mary tries to make polite/snarky conversation with one of the neighbours, asking him if he enjoyed Grace’s Purge party. The neighbour quietly tells her to go f*** herself, but when she says she didn’t hear him, whispers that the party was fine. It’s getting close to 7am when the Purge ends. Grace, seeing her opportunity for revenge disappearing, tries to grab a gun, but Mary beats her face against the table breaking her nose and tells them that they’re going to sit in mother-f***ing peace until it’s over.
The alarm then goes to announce that the Purge is over. Mary and the Bloody Stranger let the neighbours leave. The Bloody Stranger hands the gun back to Mary, and she asks him if he’s going to be okay. He nods and leaves. The surviving Standins stand in their doorway, watching the neighbours return to their homes and surveying the damage down to their property. It’s not clear what they will do next. Reports of the number of deaths and crime committed during the Purge then play over the end credits.
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