Linguist Melisande Stokes is hired by a self-described “shadowy government agency” to translate dozens of documents with provenances ranging from across the world and various eras. These documents—many of which make part of the text of the novel—all indicate that magic was once widely prevalent, but that it gradually disappeared with the rise of the Age of Enlightenment and, especially, the Industrial Revolution. In fact, Melisande and Tristan Lyons, the agent of D. O. D. O. who recruited her, eventually identify the end of magic as having occurred simultaneously with the snapping of a particular photograph, which recorded a solar eclipse on July 28th, 1851.

Melisande and her colleagues—some friendly, some not—engage in extensive research, both practical and applied. This research, as well as many other aspects of the novel, is presented in an interlinked set of letters, diary entries, slide presentations, interviews, memoranda and more, all produced by an array of characters from the past and the present (and from both).

Armed with that knowledge, and with a Schrödinger’s cat-like device designed by a disgraced theoretical physicist once employed by Harvard University, Melisande eventually learns that the agency she works for is the Department of Diachronic Operations. D. O. D. O. is dedicated to the return of magic and their work depends upon magical spells, first cast by a mysterious Hungarian witch who claims to have met Melisande in the middle of the 19th century, and then by an ever-growing number of witches brought forth from the past by a combination of witchcraft and technology.

As it becomes more obvious that changes to the timestream are accelerating towards a deadly “Diachronic Shear,” a number of witches attempt to subvert D. O. D. O. to their own ends, which includes stranding Melisande in London at the time of the Great Expedition at the Crystal Palace, a gathering of technologies and scientists that formerly signaled the end of magic, but now might signal temporal catastrophe.