The setting of this novel is futuristic supercity called Verity, with strict borders, infested with monsters, and ruled by two opposing forces.
Conflict: Man vs. Self
August is the adopted son of one of the leaders of the city, and has a very loving family. He is a monster who can steal people's souls with music, but he appears human to everyone, including himself. He is the protagonsit. He strives constantly to prove to himself that he is human enough.
Kate is the daughter of the other leader of the city. She was sent away when she was young to boarding schools by her father, and he hardly pays her any attention. She has a sweet nature, but as she grows older, she begins taking pains to prove to her father that she is as cold and monstrous as he is.
The conflict of this novel can be understood as a struggle between August the protagonist's identity as a monster and his humanity. His brother Leo encouraged him to be more monstrous and his sister Isla encouraged him to be more human. Both of his siblings are monsters also.
The resolution of the novel involves August accepting that he will never be human and vowing to serve his family's cause. He kills his brother Leo, but decides that Leo was right about August's true nature.
While in a groggy state, Kate relives the night her mother died and realizes that her mother was actually killed by her father's monster slave, Sloan.