The book starts off with Fiona Tarsis waking up in an abandoned house. She does not know why she was asleep or how she got in the situation. She realizes that there is a mark on her right hand and instinctively covers it up with concealer. As she leaves the house and wanders the streets, she gets caught by multiple citizens with guns, realizing that they are after her because she might be branded on the right side of her hand. As she tries to understand what is going on, she realizes that she is on the wrong side of the wall. Fiona meets a girl named Arrin who gives her resources and protects her.
She takes Fiona underground and teaches her how to survive in the world of beasts and monsters on this side of the wall. Fiona learns never to reveal herself as a female. Arrin says Fiona owes her and Fiona help's rescue Arrin's brother. Arrin's brother, Bowen, turns out to be Fiona's classmate. Fiona ends up running away with Bowen because of their love and learning about her family, realizing that her sister is alive and healthy. Fiona soon gets discovered when the militia breaks into an abandoned hotel, and Arrin and Fiona get pulled into a trap. They get thrown into the mix with beasts where they are bought and sold, and Arrin is told that she will be set free if she kills Fiona.
