America Singer is a 17-year-old girl living in the futuristic kingdom of Illéa where the society is divided into eight castes. One, the highest caste, contains royalty, while Eight is made up of the poorest people. America and her family are Fives, the artist caste. While it is possible to climb castes, it is difficult. Women can marry into a caste higher than their own, but men cannot climb castes. Instead, a wife must join her husband's caste. It is atypical for women to marry down. America receives a notice inviting her to register for the Selection, a televised marriage competition for the prince of Illéa, Maxon Schreave.
The Selection is how wives are chosen for Illéan princes. America's mother is ecstatic about the possibility, but America has no interest in entering the competition because she has a secret boyfriend, Aspen, a Six, a member of the servant caste below hers. America meets Aspen in her backyard tree house, their secret meeting place, and surprises him with leftovers she has saved for his dinner. The two kiss passionately but stop short of becoming physically intimate.
Pregnancy out of wedlock is punishable by imprisonment, and the children from such unions are cast into the street to become Eights. Aspen convinces America to enter the Selection, telling her that if she does not try because she's in love with him, he couldn't forgive himself for robbing her of the opportunity of a better life. America finally agrees, knowing that Aspen has to worry about his own mother and siblings. She does not want to be one more person he worries about.
America's mother lets America keep half of her earnings as a musician if she enters the contest. America is excited to keep the extra money, as that means she can save up for a wedding and life with Aspen. America goes with her mother to the local government office where she turns in her application and gets her picture taken.
While registering for the Selection, America sees Aspen's mother, who tells them that she thinks Aspen is secretly in love with someone and saving up to propose to her. America is ecstatic, and that night, she prepares a nice meal for Aspen in their tree house. Instead of being happy, he is embarrassed. He tells America that he should provide for her, not the other way around, and he breaks up with her.
A few days later and to her surprise, America is selected as one of the 35 participants in the Selection. Her participation includes a small, weekly stipend for her family, and America knows the money will go a long way in keeping her mother, father, younger sister and brother fed.
When America leaves to go to the palace, the town throws her a send off party. She sees Aspen with another girl. She is angry and sad that she has been replaced so quickly.
America travels across the country to the palace and easily makes friends with another contestant, Marlee Tames, and enemies with Celeste Newsome. Celeste is a Two, who is obnoxious and believes she is most fit to be queen of Illea. While America does not want to win the competition, she wins the favor of the palace staff by being kind and thoughtful and the hearts of the country with her unique personality.
America feels overwhelmed but finds comfort in the palace gardens. After a chance meeting with Prince Maxon in the gardens on the first night of the competition, she realizes the gardens are Maxon's refuge as well. America is very rude to Maxon during their first meeting but apologizes to him the next day and strikes a bargain with the prince: She will be his friend and help him choose his wife if he keeps her in the competition so her family can get money and she can be far away from Aspen.
They meet regularly in the gardens, and while America tells Maxon that she is not interested in marriage, the two strike up a friendship. Eventually, America even confides in Maxon about Aspen breaking her heart.
America is treated poorly by most of the other girls in the competition. At first their treatment makes her feel sad, but after Marlee tells her it is just a form of intimidation, America gains confidence. Soon she earns the ire of the other girls because Maxon takes a special interest in her. Maxon quickly dismisses eight of the girls he feels no connection with, and the competition is soon narrowed to 27 girls.
There are groups of rebels that oppose Illéa's government, and they attack the palace. While under attack, America gives Maxon advice on how to calm the other girls who become hysterical. While America handles the attack with grace and composure, some of the other girls do not handle the stress of the situation well. Slowly, America begins falling in love with Maxon, and she starts to feel jealous of Maxon spending time with the other Selected.
To America's surprise, she sees Aspen in the palace as a new member of the guard. Since America left home to join the Selection, Aspen was drafted into the army where he tested very well. He is now assigned to the palace.
Aspen's appearance confuses America's feelings, especially after she has an argument with Maxon over Celeste's poor treatment of other Selected. This leads to America questioning her ongoing participation in the Selection. She is angry with Maxon, so when Aspen sneaks into her room to see her, she begins to think that she may still be in love with him.
Aspen apologizes to America and tells her that he never stopped loving her, despite breaking up with her. He was never with another girl. He was only helping a friend, and America misunderstood what she saw. America forgives Aspen and the two kiss. Having a romantic relationship with someone other than the prince during the competition is considered treason, which is punishable by death. America uses her anger at Maxon to fuel her disregard for the rules, despite the possible consequences.
After another attack from the rebels that leads to some of the Selected choosing to leave the competition, Maxon narrows down the girls to the top six, calling them The Elite. When America ends up among the six picked to stay, she is surprised at how relieved she feels and realizes that she does have feelings for Maxon and could see herself happy with him.
Maxon comes to America's room and tells her that he is under immense pressure from his advisers to choose a wife based on social and political alliances that can be formed with the girl's family. America asks why he chose to keep her, and Maxon tells her that he has deep feelings for her. If he knew that she returned his feelings, he would end the Selection immediately and declare her the winner. He asks her if there is a chance she will return his feelings. She tells him it's possible, and they kiss.
When Aspen sneaks into her room later that night, America tells him that she cannot continue their romantic liaisons. Instead of getting discouraged, Aspen tells America that he will fight to win back her love. America realizes that she is not staying in the Selection for Maxon or Aspen, but for herself. She is at peace knowing she is right where she is supposed to be, among the Elite.