In the near future, overpopulation has depleted the world’s resources, so the U. N. decides that countries must cut their population by 5% every year.
In the USA, all students take a standardized test once a year and those with the lowest scores are executed. The test is called the 10-241 but people have nicknamed it The Thinning. In Texas, students are studying for the test, which will start in eighteen hours.
High-schooler Laina is mentoring Simon while her younger siblings play behind them. Simon is frustrated because he doesn’t understand enough and pays Laina a good amount of cash, which gets him special contact lenses that give him all the answers. Laina reminds him to destroy them after the test.
At that moment she receives a call from the hospital saying that her mother is trying to get discharged against medical advice. Laina asks the doctor to buy time and rushes to the hospital with her siblings, where her mother has already signed all the discharge paperwork. The doctor pulls Laina aside and turns down her money, revealing her mom has gotten worse and no treatment will help her now.
In the evening with only twelve hours left for the exam, the governor’s son Blake tells his bodyguards he’s going to the kitchen for a bite, but he actually sneaks out of the mansion and gets in his car to meet his girlfriend Ellie. The mood gets awkward when they remember they may die in the morning, so they decide to distract themselves. They sneak into another mansion and use the pool for a while.
Afterward they return to the car to make out, but Ellie can’t get in the mood because she’s scared of the test. They’re suddenly interrupted by a bodyguard, who drags Blake back to his house. Governor Dean scolds Blake for wasting his time the night before the test, pointing out he should be studying now so he can have fun later.
He thinks Ellie is a distraction that Blake can’t afford until he graduates next year, but Blake doesn’t look convinced. Dean gives him a hug anyway. Three hours before the test, guards in black surround the school with fences, barbed wire, and cameras.
They also patrol the perimeter to make sure nobody runs away. Simon arrives early and another student accidentally bumps into him, causing him to drop his contact lenses. He tries to find them, however a guard tells him to keep moving.
As the students arrive, they share a sad goodbye with their parents, then the guards check their bags and scan their bodies to prevent any cheating tricks. Blake approaches Laina and asks to buy the special lenses, but Laina tells her she already sold the last ones. At that moment a guard finds a student with invisible answers painted on his arm and the boy immediately runs away.
This is the first time someone makes it past the guards, so Laina’s friend Kellan hacks his father’s account to access the street security footage, watching how the guards surround the guy and catch him with a net. Then Kellan makes a copy and sends it to a reporter. Once everyone is inside, the guards put the school under lockdown.
Then the students go to their respective classrooms and Miss Birch tries to be as comforting and encouraging as possible, telling them there’s always a chance. Mister Glass reminds them of the rules: they have two hours to finish the test, their eyes must never leave their tablets, and all grades are final. At that moment the test finally begins and everyone concentrates on answering the questions on the tablet, but it’s clear many of them are struggling.
When the test is finally over, the students must drop their tablets and the guards immediately go over the results in their computer. The conclusion arrives in just a matter of minutes and the teachers are heartbroken to read the list with the lowest scores. With each name mentioned, the guards take the student away, dragging them away when they try to resist or even beating them up when they try to escape.
Blake, Kellan, and Laina passed, but Ellie didn’t and she’s taken away. The students are allowed to have a break and Laina sees Simon is also being taken away. A desperate Blake calls his dad to ask him to save Ellie, but Dean refuses to play favorites since the law is the same for everyone.
Furious, Blake pushes a bunch of people to reach the guard, pushing him too before grabbing a fire extinguisher and hitting him with it. Then he jumps on the second guard and wrestles him while telling Ellie to escape. She immediately runs away but the other students block the corridor and the guards catch up to her, taking her to her death while a screaming Blake is dragged away.
A year passes and the next test will be in twenty-four hours. Sarah can’t handle any more studying and goes to see Glass, who agrees to give her a good grade in exchange for doing the dirty with her. Meanwhile Birch visits Laina and her siblings, who recently lost their mother.
Birch has brought food and offers her support because she knows the little ones are having their first test this year. On her way home, Birch sees Sarah leave Glass’ car. At the governor’s mansion, Blake records a video on his tablet, saying that if someone is watching this, it means he’s dead.
Three hours before the test, Laina takes her siblings to the school, quizzing them on the way to make sure they’re ready. Dean tries to be supportive of Blake before he leaves, but Blake doesn’t buy it and tells him to save it for his speeches. A bodyguard notices Blake dropping an envelope in a mailbox on his way to the school, so he steals it and takes it to Dean.
Moments later, the school is put under lockdown and all the students take their seats. In the kindergarten class, the teacher plays an animated video full of propaganda that explains why the test is important to save the Earth. Other countries kill the elderly or restrict the amount of annual births, but the USA concentrates on keeping the smart people so it can be great again.
Then the students are given the tablets and the test begins. Meanwhile Dean finds Blake’s video inside the envelope and learns that he’s going to fail the test on purpose so his father can watch his son die because of the system he defends so much. Dean immediately asks the bodyguards to bring Blake home, but nobody can leave the school after lockdown.
After making his bodyguard leave the room, Dean calls Mason, the head of the school’s security operation. In the classroom, Blake answers everything fast and wrong, allowing him to finish early. Glass finds this strange.
In the kindergarten class, a kid raises his hand and asks for help, but the teacher refuses to assist him because a guard is watching. Eventually the time runs out and everyone puts down their tablets. This time the results are calculated by Mason alone instead of a group of guards.
Soon the teachers start calling the names and the kid that asked for help is taken away. So is Sarah, who screams as she realizes Glass lied to her. Kellan and Laina’s siblings pass and so does Blake, who doesn’t understand what’s happening.
Birch cries as she reads Laina’s name, not believing it because she’s the best student in her class. Meanwhile Mason calls Dean to confirm he made the switch. A shocked Blake watches the kids being taken away and Birch runs to a guard, asking him to double-check the results since it’s impossible for Laina to fail.
The guards call Mason and they’re told everything is fine, so Laina must leave with the others. As Birch says goodbye, she secretly gives Laina her keycard, reminding her there’s always a chance. Afterward the school throws a party for the people who passed and Birch notices Glass flirting with another student.
At the same time Dean is on a public event and his speech is played at the party. The whole crowd is cheering for him as he mentions all the things he’s improved in the state and comments on how decisions are had to make but also necessary. He thinks the Thinning isn’t barbaric, instead he calls it innovation.
Dean also announces his candidacy for president and a disgusted Blake leaves the party. In a private hallway, the failed kids are told to take off their clothes for decontamination and hit when they don’t obey. Laina puts the keycard in her mouth to keep it hidden.
Then they’re given prison-like clothes and taken to another room where they’re handcuffed to the chairs. Nearby, Blake reaches the corridor and pretends to feel very sick. When a guard checks on him, Blake jumps on the man and knocks him out with his own weapon.
At the same time, Kellan gets into his father’s account again and watches the failed students on the security camera, but it’s too painful and looks away. After hiding the unconscious body, Blake waits for Mason to leave the control room and he sneaks inside. He tries to use the computer but it asks for a password.
As the guards get ready to inject the students, Blake shuts down the power in the whole building by messing with the power box while Mason finds the unconscious guard. The electronic handcuffs begin to fail and the students try to escape, letting chaos take over the room. As the guards and the students fight, Laina uses the chance to escape by using Birch’s keycard on the door.
Soon Mason returns to the control room but Blake is already hiding in the vents. Mason sees the system is down and tells the guards to bring Kellan’s dad to fix it. At the party, the guards inform everyone that it’s just a regular power outage and take Kallen’s dad to the control room.
While the man works, Mason goes to check on the students, who have been handcuffed again. Mason reminds the guards to make a headcount and they notice someone is missing. In the corridor, Ellie almost bumps into a guard but she hides among the lockers just in time.
Then she tries opening another door with the keycard, however it’s stuck. At that moment the guard finds her, so Ellie uses the darkness to pretend she’s Birch. As soon as the guard turns around, Ellie jumps on him and tries to fight him, but the guard overpowers her and starts kicking her.
Suddenly Blake shows up and takes over the fight, using the element of surprise to knock the guard out. Afterward Blake admits he failed on purpose and Laina realizes something is off with all the results. Blake then activates the alarm to send all the guards to the corridor while he and Laina hide in the vents.
Mason announces the school will stay in lockdown until they find Laina, not caring about the fact the kids are supposed to be reuniting with their parents by now. In the vents, Blake judges Laina for selling the lenses to the desperate kids because he sees it as taking advantage of their fear. Laina has to explain she only did it to pay for her mother’s treatments, and Blake immediately apologizes.
She also admits that she used to have a crush on Blake. At the governor’s home, a reporter asks Dean about the prolonged shutdown at the school, which Dean didn’t know about. He pretends everything is under control and after the interview he checks the news to discover Kellan’s reporter friend is outside the school.
She’s covering the crisis and interviewing parents who are worried about their kids’ fate. Meanwhile Mason makes all the teachers prove they still have their keycards. When he’s about to check Birch, he’s interrupted by a call from Dean, who furiously yells at him to solve the problem quickly.
Back to Blake and Laina, they continue to move through the vents when suddenly the surface breaks and Blake falls through, landing in the pool. Laina immediately jumps in and swims quickly to rescue Blake, who is unconscious. Fortunately Laina knows CPR and manages to wake him up.
Then they head to the lockers and turn around while they change into dry clothes. Laina finds another vent so they can keep going. At the party, some students try to leave only for the guards to beat them up, so Kellan starts recording and sends it to the reporter.
Minutes later, Laina and Blake reach the corridor outside the server room and the keycard falls from Laina’s pocket. The guards hear a noise so Blake immediately covers Laina’s mouth and they wait for the men to pass by. Thankfully because of the darkness they don’t see the key.
Next the duo goes to the science classroom, and this time the noise they make as they descend is definitely heard by a guard. Putting a few objects together, Laina makes a magnet rod to fish for the key. At that moment the guard arrive and starts pounding on the locked door, so Blake sends Laina back into the vents while he stays as a distraction.
Blake tries hiding behind the desks, but the guard bursts in and quickly finds him. Soon only one of them is unconscious on the ground. In the vent, Laina waits for the guards to move and lowers the magnet.
She manages to catch the key and raises it, only for a guard to suddenly grab it. However he also lets go of it and Laina realizes this is Blake, who stole the uniform from the guard he knocked out. Meanwhile the guard from the previous fight tells Simon there are two teenagers on the loose and one of them used the name Birch.
In another room, the teachers are still waiting. Birch pretends to flirt with Glass and steals his keycard. When Mason comes to check on her, Birch presents Glass’ keycard as her own.
The guards notice Glass is missing his key and he gets detained while the other teachers are finally allowed to leave. Mason starts beating Glass up as he asks where to find the escaping teens. Back to Laina, she takes the vents to the server room and tries using the computer, but the test program asks for Mason’s password.
She sends a message to Kellan, asking for help. At that moment Kellan’s dad finishes his repairs and the power comes back. Kellan gets wifi and receives Laina’s message, so he replies with a video of Mason using his password earlier.
Laina copies it and accesses the test system, first checking on her siblings to confirm they passed. Then she checks her own results and sees she got a 98% but still failed while Blake passed with 15%. The same happened with Ellie, who failed with a score of 88%.
It seems Dean has been messing with the test results for a while. Speaking of Dean, he appears on a press conference and claims that Laina killed two guards after failing the test, but the guards are working hard to keep everyone safe. At the same time Kellan sends his reporter friend the video of the guard beating up the students that tried to leave, which immediately gets played on national TV.
Laina guesses this is Kellan’s doing and sends him some screenshots of the manipulated test results. Sadly Mason finally sees her on the security cameras and raises the alarm, so she starts running. Mason and his guards quickly surround her and she tries to say she has proof she actually passed, but she gets arrested anyway.
Meanwhile Blake finds the room with the students and pretends he has orders to move them. The other guards find it strange since they weren’t informed, so Blake has no choice but to trigger a fight. He barely manages to throw some punches before the guards surround him and overpower him too.
At the party, Kellan receives the screenshots and he sends them to the reporter, who immediately appears live on TV to announce her discovery. She comments on how suspicious it is that a politician’s son got a benefit from this, and at the mansion Dean’s team panics. They think he should shut the mission down and let Laina go, but Dean says he has one final move.
Soon Laina is handcuffed with the rest of the failed kids and the guards get the injections ready. Mason calls Dean to ask for authorization to proceed, but Dean stops him, saying that the list needs to be revised. The students that actually passed the exams are released and Laina runs to kiss Blake, who is tied up.
Some guards drag her away while others go to the party to bring the students that actually failed the exam. This includes Blake, who is also handcuffed with the others. One by one, the students get a mysterious injection and they pass out.
Now the school is finally opened and the parents reunite with their children. Laina runs to meet with her siblings, who have been watched over by the teacher. The parents that don’t get their kids back have a breakdown on the spot.
Soon Dean appears on TV again and pretends Mason was behind everything. He puts up a sad act about his love for his dead son, still clarifying not even him is above the law. Sometime later, a truck arrives at an illegal tech factory.
To Blake’s shock, he wakes up alive and well because the injection had only been a sedative. The other students are also alive and when they look around, they discover failed students from previous years working at the factory. Blake gets another shock when he sees a very alive Ellie.
