London 1817. Maggie Collins, born into slavery in Maryland, whose mathematical genius and strength of mind can match those of a goddess, must build the world's most powerful and sophisticated machine-- to free the lost land of Yount from the fallen angel Strix Tender Wurm. Sally, of the merchant house McDoon, who displayed her own powers in challenging the Wurm and finding Yount in The Choir Boats, must choose either to help Maggie or to hinder her.

Together-- or not-- Maggie and Sally drive to conclusion the story started in The Choir Boats-- a story of blood-- soaked song, family secrets, sins new and old in search of expiation, forbidden love, high policy and acts of state, financial ruin, betrayals intimate and grand, sorcery from the origins of time, and battle in the streets of London and on the arcane seas of Yount.