In the near future, overpopulation has resulted in such extreme shortages of global resources that the United Nations has mandated all nations reduce their populations by 5% each year. Some countries take to euthanizing their elderly, while others implement one-child policies. The United States of America implements their own population control measure in "10-241", a standardized test for all students grades 1 through 12. Those who pass the test proceed to the next grade or graduate, while those who fail are euthanized. This test becomes colloquially known as "The Thinning".
Laina Micheals is a middle-class high school student in Austin, TX, whose mother is in failing health. Even though she is a bright student who has no worries about passing the coming Thinning, her younger brother, Nate, is having trouble studying. To that end, she gives Nate a "study guide": a set of contact lenses that provides answers to tests. In the same city is a boy named Blake Redding, the son of Governor Dean Redding, who is similarly intelligent, but rebellious and dating a girl named Ellie, whom his father disapproves of.
The day of the Thinning comes. Laina and Blake both pass, but their relief is cut short when Laina discovers that Nate, who lost his contact lense cheat on his way to school, has failed, as well as Ellie. Blake desperately calls his father to ask him to save Ellie, but Dean refuses, stating that no one is exempt from the law. Blake tries to create a diversion so Ellie can escape, but fails and is forced to watch as Ellie is taken away to be executed.
The following year, Laina, now the only caretaker for her young siblings, tutors her sister Corrine: she is now in the first grade and will be undergoing her first Thinning. Blake, meanwhile, has become cold towards his father. On the day of the Thinning, he records a video and places it in an envelope in a mailbox on his way to school.
One of Dean's agents intercepts the package. The video is a recorded suicide note, as Blake plans to intentionally flunk the test and allow himself to die to send a message to his father, who plans to announce his candidacy for President, about the system he supports. Dean calls the head proctor of his son's high school's Thinning, Mason King, and instructs him to alter the test results so his son would pass.
When the Thinning test ends, Blake is shocked to find that he had passed, in spite of his intentionally trying to fail the test. Equally shocked are Laina, who has failed; and her teacher, Ms. Cole. Before Laina is taken away, Cole slips her her ID badge, which Laina hides in her mouth as she is taken to the Thinning chamber along with other students who had failed.
Blake, in desperation, sneaks into Mason's office to pull the test scores, but his efforts are rebuffed by the need for a password. As a Hail Mary, he pulls several cords from a fuse box, causing power in the school to go out and forcing the facility into lockdown. Laina takes advantage of the situation to make her escape, using Ms. Cole's ID badge to escape the death chamber and run.
Blake and Laina, who are both acquaintances, meet as they make their way through the school. Together, they plan to discover what really happened that caused Blake to pass and Laina to fail. Blake subdues one of the school guards to disguise himself, while Laina makes her way to the server room and, with the help of a tech-savvy friend, Kellan, who is leaking the increasingly chaotic situation at the school to the media, gains access to the school's servers. To her horror, she discovers that, in spite of having a score of 98%, she was listed as having "failed" the test while Blake "passed" with a score of 15%. Several other students had also been falsely registered as having "passed" or "failed" in spite of their test scores. In addition, during last year's test, Blake's girlfriend Ellie had a score of around 80%, but was listed as having "failed".
Ellie sends evidence of the tests being rigged to Kellan, who subsequently leaks the test results to the media. Unfortunately, the power in the school is restored, and Laina is caught on camera hacking into the computer. She is apprehended by Mason as Blake is also captured in a failed attempt to rescue the other students in the death chamber.
With the news now reporting on his corruption and jeopardizing his candidacy, Dean Redding orders Mason King to halt the Thinning and revise the scores. As such, Laina, along with the other students who had actually passed the test, are released. Unfortunately, as per Dean's instructions, the students who actually failed the test are to be put to death, including Blake, his own son.
As Laina is reunited with Corrine, who had passed the test, Dean holds a press conference where he pins the blame of his own manipulation of the Thinning on Mason, and contemplates on the death of his son.
Little does Dean know, Blake isn't dead. The "lethal injection" he and the other failing students received was actually a sedative. They are taken to a secret underground facility responsible for making the tablets that the Thinning test are conducted on, with Blake discovering that other failed students, including Ellie, are not killed, but instead used as slave labor...
