What would you do if you had to leave everything you knew behind? If what made you perfect also made you an outcast? If your twin, once your only friend, was now your worst enemy? Since the blast that reshaped the earth, only twins are born. The imperfect one of each pair is branded at birth and sent away. Twins share nothing but the moment of their death: when one dies, so does the other. But Cass and her twin Zach cannot be separated. In this scorched and broken world, Cass's bond with her brother may be the most dangerous thing of all.