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 realises her love for Maxon one night but she does not reveal this. She encounters Celeste who is miserable at the fact that America has gained so much popularity. She apologises to America for her behaviour and they two become friends.
Aspen helps America to visit George at his 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. 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 publicised. She realises 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 jewellery.
The King urges Maxon to remove America from the Selection but she has become too popular. The King also realises that America is advantageous to keep because she managed to ally Illea with Italy. 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. Her father left her a letter to "look unto the North Star" and she realises that he was a Northern rebel.
The Selection gets narrowed down to two: America and Krisse. America notices star necklace on Krisse and realises 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 and he leaves upset. America talks t Aspen, and Maxon witnesses this and realises that America and Aspen had a relationship. Maxon's anger leads him to retract his marriage offer to America.
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.