The book opens with a flashback: Ten-year-old Lucie Herondale is at Herondale Manor, her family home in Idris, the home country of the Shadowhunters. She sneaks out to play in the surrounding Brocelind Forest one night. Panicking when she can’t find her way home, Lucie stumbles into a deep hole dug into the earth. A pale boy, who looks 16, appears at the head of the pit, warning Lucie not to yell as it will alert the faeries; faeries abduct children to use as slaves. When he retrieves Lucie from the pit, she introduces herself, but the boy offers Lucie no name in return. Six years will pass before Lucie sees him next.

In the present day of 1903, Lucie’s 17-year-old older brother, James, and his friends patrol the London streets for demons. James is a Shadowhunter, a warrior of angelic descent with special powers and the ability to wield weapons made of adamas, an indestructible metal. Shadowhunters, also known as Nephilim, live in the Shadow World, the unseen realm in which supernatural creatures—such as Downworlders—dwell. Nephilim keep this world hidden and peaceful and also protect humanity from demons, which are evil, destructive beings that can travel across dimensions.