Tessa is finally safe in the magical underworld of Victorian London alongside the Shadowhunters. However, this safety proves to be temporary when threatening forces within the Clave conspire to overthrow her guardian, Charlotte, from her position as head of the Institute.

If Charlotte loses her position, Tessa will be on the street - and she will be easy prey for the mysterious Magister, who wants to use Tessa's powers for his own dark purposes. With the help of the charming, self-destructive Will and the passionate devoted Jem, Tessa discovers that the war the Magister has unleashed against the Shadowhunters is deeply personal.

He holds them responsible for a tragedy that shook his life many years ago. To unravel the secrets of the past, the trio travels from fog-covered Yorkshire to a mansion that hides within it unspeakable horror, from the slums of London to an enchanted ballroom, where Tessa discovers that the truth about her origins is more ominous than she had imagined.

When they encounter a clockwork demon with a warning for Will, they realize that the Magister knows their every move - and that someone close to them has betrayed them. Tessa's heart is increasingly drawn to Jem, yet her desire for Will, despite his dark mood, continues to disturb her.

But something is changing in Will - the wall he has built around himself is crumbling. Could finding the Magister free Will from his secrets and give Tessa the answers to who she is and what she was born to do? As the search for the Magister and the truth brings them face to face with danger, Tessa learns that when love is mixed with lies, it can corrupt even the purest heart.