Upon graduating from school, Malencia "Cia" Vale of the Five Lakes Colony is chosen to take part in the Testing, a rigorous yet mysterious process by which young adults are selected to attend University. At first, she is overjoyed at the news, but her father privately tells her of his own vague memories of the Testing. He recalls explosions, screams, and death. He warns Cia not to trust any of the other candidates. Allowed to take three items from home, she chooses her brother Zeen's transit communicator, as she believes it will help her along the way.
Cia, along with classmates Tomas, Zandri, and Malachi, and testing official Michal Gallen, leave Five Lakes Colony for Tosu City, the capital of the United Commonwealth - what's left of the United States after an apocalyptic war and assault of natural disasters. Cia notices their skimmer has hidden cameras that catch their every action and word. She makes a point to be on her best behavior at all times. Cia shares this information with Tomas, and they agree to look out for each other over the course of the Testing. Once the group reaches Tosu City, Cia meets the other 108 Testing candidates vying for 20 open seats at University.
The first round of Testing is a number of written exams that test the candidates’ history, math, and reading skills. Cia thinks she performed adequately, but her roommate Ryme tries to make Cia feel inferior. Later, Cia finds Ryme has committed suicide in their dorm room. Dr. Barnes, the highest Testing official, is apathetic towards Ryme's death, asserting the Testing served its purpose at weeding out the weak. Cia keeps her composure for the cameras.
During the second round of testing, the candidates perform deadly hands-on experiments. After ingesting a poisonous plant, Malachi accidentally gets shot in the eye with a nail and dies.
The third round of testing consists of a group problem solving exercise. While she waits for her turn, Cia realizes that team member Roman has planned to trick the others into disqualifying themselves. Though it is too late to save the other female candidate, she is able to warn Brick. After completion, Cia's finds her caution was correct, as the female candidate does not return to the Testing.
The fourth round of testing is a survival challenge. Candidates will have to find their way back to Tosu City, by any means necessary. Tomas concocts a plan to meet up with Cia at the tallest building left in the ruins of Chicago, the starting point of the exam. When Michal chaperones Cia to the site, they discuss the harsh measures taken during Testing, and he mentions those critical of the government believe that Cia will be able to help them from the inside.
Once the fourth round of testing begins, Cia immediately looks for Tomas. However, another candidate tries to kill her with a crossbow. While running away, she jumps across a ravine and almost falls into a river. Tomas saves her, and they look for a water source together. An oasis appears too good to be true - and it is; it explodes and nearly kills Tomas. Cia patches him up and they get back on their way.
The next day, Cia and Tomas find the body of a dead female candidate. Cia takes her Testing identification bracelet in order to honor the girl. That night, Cia and Tomas offer their camp to three other candidates. Cia begins tinkering with the dead girl's bracelet, and she finds a microphone embedded inside. Once they are alone again, Cia tells Tomas they need to be more careful about what they say. They also both admit they have feelings for one another. In the night, they are ambushed by mutilated animals, but escape to a demolished city in the distance.
Cia and Tomas realize the buildings have been fashioned into a long, complicated maze. They run into Will, a candidate Tomas does not trust. Once out of the city, Cia leaves the two boys to find food. A man appears on the other side of the Testing fence and throws a bag of food over for Cia. She returns to the boys but keeps the man and his food a secret.
The next day, Cia again leaves Tomas and Will in order to find a water source and let them fight out their differences. She is attacked by a group of mutated humans, and has no choice but to kill them. She makes it out alive, but is scarred by taking life. The next day, she returns to the boys and notices they are both hiding a secret. She does not press the issue, but it makes her unable to trust Tomas. Will leaves the two to travel on his own, and Cia then returns to the fence to talk with the man she saw earlier.
The man at the fence tells Cia he is fighting the Testing and its brutality. He gives Cia a vial of liquid to help counteract the truth serum she will be given during the interview portion of the Testing. Cia returns to Tomas, still keeping the man at the fence a secret. They come up to another city, which is filled with a number of mutated humans that attacked Cia earlier. Cia and Tomas slowly make their way through the city.
When Cia and Tomas are almost through the city, Brick opens fire on the mutated humans, believing he is saving Cia like she saved him in the third round of testing. Later, she and Tomas are ambushed by Roman, who tries to kill them both. Will intervenes and shoots Roman. When the three are ambushed by another candidate, Will turns on Cia and Tomas. Cia realizes it was Will who tried to kill her with the crossbow. Will manages to shoot Tomas, but Cia wounds Will before he can take her out. Will escapes in a skimmer, and Cia tends to Tomas' wounds. The next day, she drags Tomas across the finish line in Tosu City.
Once back at the Testing facility, Will confronts Cia and apologizes for trying to kill her, though he feels like he was doing the right thing at the time. He believes that the Testing was designed to force people to make bad choices to see if they could live with the decisions they made. In a final effort to conceal her feelings and keep her family safe, Cia drinks the liquid that the man at the fence gave her before she goes into her final interview.
Cia is able to hide many incriminating facts about her family and her knowledge of the Testing. Dr. Barnes accuses Cia of having distractingly strong feelings for Tomas, and is pleased to tell her that Tomas was involved with Zandri's death during the fourth round of Testing. After the interview, Cia, distraught, confines herself to the bathroom, where there are no cameras. She finds a recorder on the transit communicator, and records everything that happened during the Testing.
Cia is accepted into University along with Tomas and Will. However, the officials erase the candidates' memories, and no one can remember anything about the Testing. On her birthday, Michal gives Cia a care package from her family and congratulates her on her success. Later in her room, Cia finds the recording and listens to her own account of what happened during the previous weeks.