In a city drowning in fog, even a monster can be a sanctuary.

In the climate-controlled silence of the British Library, manuscript conservator Cordelia Elliston lives a life of careful repairs and quiet order. But the charred diary she finds in a private estate clearance refuses to be fixed. The leather is fever-hot. The ink moves like living shadow. And when the page pulses beneath her fingers, Cordelia is pulled through a crack in the world.

She wakes in the sulfurous, batswing-lit London of 1888. It is a city paralyzed by murder, but the killings are merely a symptom. The "Troth"—the ancient boundary that keeps the dark at bay—is failing, and the city’s sanity is leaking through the cracks.

Below the rotting timbers of Blackfriars Bridge, Cordelia finds the source of the rot. Sunder. A guardian of the Troth, a being of elemental iron and ancient bone, chained for forty years by a man who would break the world to reclaim his own heart.

To save the city, Cordelia must do the one thing her trade forbids: she must destroy an ancient thing to find the truth inside.

But as the lines between the present and the past begin to blur, she realizes that the life she left behind is a lie, and the only truth left is the monster who knows the shape of her soul better than he knows his own chains.