When Jace and Clary meet again, Clary is horrified to discover that the demon Lilith's magic has bound her beloved Jace together with her evil brother Sebastian, and that Jace has become a servant of evil. The Clave is out to destroy Sebastian, but there is no way to harm one boy without destroying the other.

Sixteen-year-old Clary continues trying to make sense of the swiftly changing events and relationships in her life as she becomes further involved with the Shadowhunters and their pursuit of demons and discovers some terrifying truths about her parents, her brother Jace, and her boyfriend Simon.

Still pursuing a cure for her mother's enchantment, Clary uses all her powers and ingenuity to get into Idris, the forbidden country of the secretive Shadowhunters, and to its capital, the City of Glass, where with the help of a newfound friend, Sebastian, she uncovers important truths about her family's past that will not only help save her mother but all those that she holds most dear.

No one knows what Nikolai Lantsov endured in his country's bloody civil war. Now enemies gather at his weakened borders, and the young king must find a way to refill Ravka's coffers, forge new alliances, and stop a rising threat to the once-great Grisha Army.