To save her mother's life, Clary must travel to the City of Glass, the ancestral home of the Shadowhunters - never mind that entering the city without permission is against the Law, and breaking the Law could mean death. To make things worse, she learns that Jace does not want her there, and her best friend, Simon, has been thrown in prison by the Shadowhunters, who are deeply suspicious of a vampire who can withstand sunlight.
