For years, Sia's hidden herself in the gnashing forest, dodging her father's bounty hunters and ignoring the nameless magic in her veins. When she receives a letter from an estranged lover, she returns to the coast she used to call home, within reach of her father's magical city hovering above the smog and the arranged marriage waiting for her. This place is not somewhere Sia wishes to linger-the land is noxious and filled with rebels, witches, and other humans who become walking corpses when they die. What was intended to be a brief visit turns into a death-bed promise anchoring Sia to Gwaith House for the foreseeable future. She agrees to look after her first love's human child until her widower proves he can care for his daughter himself.

The thing with promises is: Sia can't break them. No seraph can. At least, not without severe consequences.

While navigating the complex feelings she develops for the angry widower and her own grief, Sia becomes entangled in a mysterious string of dismembered corpses roaming the smog and a strange detective who refuses to leave her alone. As the desperate undead draw closer and closer to the child she is sworn to protect, Sia races to find the murderer responsible for creating them. She needs to figure it out quickly, or she may risk falling right back into her father's scorching hands.