America Singer is a sixteen-year-old girl living in Illéa. Illéa is a newly founded United States after the original United States lost World War III to China. Illéa’s people are organized by a numerical caste system. America’s family are Fives, which means they are required to become artists and musicians, regardless of their talent or desire. Much of Illéa is poor, and the country is constantly under attack from two rebel factions—one from the North and one from the South—both of which hope to overthrow the government. One day, America receives an invitation to apply to the Selection, a contest in which thirty-five young women between the ages of sixteen and twenty, from any caste, live at the palace while competing to win the Prince’s hand in marriage. The contestants are paid to participate, and America’s mother pesters her to apply since the stipend will help the family financially. America feels repulsed by the idea, mostly because she already has a boyfriend.
America has been secretly dating a boy named Aspen for over two years. They can’t reveal their relationship, however, because Aspen is a Six, the servant caste. If they marry, America will lose her chances at lifting her family out of poverty. America and Aspen could even be jailed if caught together. Surprisingly, Aspen encourages America to apply, explaining that he doesn’t want her to lose a chance to marry a Prince. America experiences increasing pressure from her mother, who reluctantly offers to let America keep half of her earnings from her musical gigs if she applies. Thinking she can now save money for her and Aspen to marry, America agrees to apply.
Much to everyone’s astonishment, America is selected. Aspen, however, reacts emotionally and breaks up with her. In a moment of vulnerability, America tells Aspen that she was hoping to marry him to discourage him from leaving. He says he can’t marry her and rushes off. At the town’s send-off celebration, America sees Aspen with his hand around another girl’s waist. America departs for the palace in rage. She feels confused and hurt, and she wonders how her life changed so quickly.
America’s first meeting with Prince Maxon doesn’t go well. She lashes out at him, saying she doesn’t want to be there and loves someone back home. Maxon reacts with compassion, which surprises America. She offers to stay in the competition as his friend and act as an insider. He gratefully accepts and promises to keep her in the competition until the end.
Over the next several weeks, America’s and Maxon’s feelings grow for each other. While the other girls alter their appearance to win Maxon’s attentions, America refuses to change who she is or how she looks. She speaks her mind, even when her opinions clash with Maxon’s. America and Maxon develop a system in which they signal to each other when they want to talk, and in time, their friendship blossoms. Competition with the girls grows fierce. A girl named Celeste continually hijacks situations, including “accidentally” spilling her red drink on one girl’s white dress and tearing America’s dress when she refused to hand it over to Celeste. Some girls remain friendly, though, including Marlee, with whom America develops a close friendship.
During the competition, the palace is attacked several times by rebel forces, and America begins to realize the realities of being a royal. She grows more sympathetic toward Maxon, and she and he eventually spark a relationship. In turn, America opens Maxon’s eyes to the realities of being a commoner. He institutes a new rule where half of the girls’ stipends are donated to a food assistance program for lower castes. With the increasing attacks from rebel forces, Maxon worries about the girls’ safety and sends home all but six. He confides in America that he’d send them all home but her if she’d agree to be with him now but understands America is still unsure of her feelings. Maxon says he’ll be patient.
One day, Aspen suddenly appears in the palace: He now works as a palace guard. America feels shocked. Unaware that Aspen was once America’s boyfriend, Maxon assigns him to guard America’s door. Aspen sneaks into America’s room one night, and they finally talk. Aspen reminds America that he didn’t want to propose to her because he was afraid he’d be drafted and separated from her for years. He also claims that back at the send-off celebration, he wasn’t with another girl; he was only helping the girl to her feet after she fell. America believes him, and they passionately kiss. America remains conflicted and confused. She realizes her behavior could be considered treason. Later, when Aspen stops by her room to ask her whom she is choosing—him or Maxon—she tells him that when he broke up with her, he broke her heart. She explains that she needs time to sort out her feelings for Maxon. Heartbroken but inspired, Aspen says he’ll just fight harder for her.