The novel opens with a rebel attack. Two rebels speak with Maxon and America and America identifies her them as the two people she saw when she was hiding in the tree in the previous novel. They reveal themselves to be August Illea and Georgia Whitaker. They reveal their desire to end the caste system but also state that other rebels want to destroy the monarchy and keep people in the lowest castes. They urge Maxon to choose America as his wife because it would be good for the cause.
America realizes her love for Maxon one night but she does not reveal this. She encounters Celeste in the library who was crying about the fact that America has gained so much popularity from the talk during the report. She apologizes to America for her behavior during the selection and them two become friends.
Aspen helps America to visit Georgia and August in their home, where he tells her that three hundred Two's have been killed in the rebel attacks. As America is leaving they are attacked by the Southern rebels and America is shot in the arm. America met a girl named Paige while she was heading and they manage to escape back to the palace. America later meets with Italian princess Nicoletta and urges her to supply weapons to the Northern rebels to fight against the Southern rebels. Nicoletta agrees.
As part of the Elite test, America is made to jail petty criminals and this is to be publicized. She realizes that the king wants her to imprison a man simply become he stole some clothes for his children because he wants her to appear merciless. However, she pays off the man's debts by giving away her jewelry.
The King urges Maxon to remove America from the Selection but Maxon had convinced his father that she has become too popular. The King also realizes that America is advantageous to keep because she managed to ally Illea with Italy, a country whom they wanted to become an ally with for years. On Christmas, Maxon admits his love for America and just as she is about to say it to him, the messenger informs her of her father's death. When she reaches home she feels as though it doesn't feel like home. There Kota got mad at America and he tells the whole family that she had a secret relationship with Aspen. He threatens to tell Maxon, but America shuts him down and tells him that she will tell him herself. Her father also left her a letter to her before he passed, it said "look unto the North Star" and after looking at his weird signature, she realizes that he was a Northern rebel.
The Selection gets narrowed down to two: America and Kriss. America notices a star necklace on Krisse and realizes she is also a Northern rebel. Maxon reveals to her that he has chosen America as his wife. The next morning, Aspen comes into America's room to find her and Maxon together half-naked and he leaves shocked. America talks to Aspen while putting her hand on his chest, and Maxon witnesses this and realizes that America and Aspen had a relationship. Maxon's anger leads him to retract his marriage offer to America and choose Kris for his wife instead.
During the engagement ceremony, the Southern rebels attack. Maxon takes a hit for America, thereby saving her life. He tells Aspen to take her to a safe room and also reveals he loves her. The King and Queen are killed in the attack and Maxon is made King. Maxon re-proposes to America and she accepts. She visits Maxon in his room where he is recovering from his wound. He tells her his decision about dissolving the castes, starting with the Eights merging with the Sevens and so on. He then gives her a beautiful engagement ring, officially asking her to marry him. She happily accepts his proposal.
In the epilogue, America prepares for her wedding with Maxon. In the bonus epilogue, America reveals she is pregnant.
