Cass is a graduating senior in high school whose life turns upside down when she is diagnosed with a rare disease: Casper-Williams Syndrome. Doctors x-ray her and find that, like all other people with the disease, she has a map of a city on her bones. All her life, Cass has felt strange pains in her body, but when she goes to this city, the pains stop. Much to Cass' dismay, it is not an exciting city like London or Paris or New York, to which she has wanted to travel her whole life. But she is stuck in this new place and does her best to make the most of it. She gets a job at a diner and moves into an apartment. All the while, the disease is progressing. Cass is slowly becoming invisible to the world around her and finds it harder and harder to hold onto her own memories. Meanwhile, she also finds herself with super strength, super speed, super healing abilities, and strange visions—all of which allow her to fight the crime in the city. She takes a new name for herself: Seraph. Then arrives a sort of anti-Seraph. He is about the same age as Seraph and has the same disease, but instead of protecting the city, he wreaks havoc upon it. Seraph spends her days fighting him until she realizes that she knew him from her past. Seraph and the boy used to date in high school and they caught the disease from one another. It turns out that he even moved to this city with her, but she had long since forgotten. The boy pleads with Seraph to destroy the city with him so that they can run away and be free. It is a hard decision for Seraph. She also wonders if she should get the newfound cure for her disease, painful as it is. Eventually she decides to kill the boy, only to realize that this disease and the city will continue to control her life unless she does something.