Welcome to the Witherward, and to a London that is not quite like our own. Here, it's summertime in February, the Underground is a cavern of wonders and magic fills the streets. But this London is a city divided, split between six rival magical factions, each with their own extraordinary talents – and the alpha of the Changelings, Gedeon Ravenswood, has gone rogue, threatening the fragile accords that have held London together for decades.

Ilsa is a shapeshifting Changeling who has spent the first seventeen years of her life marooned in the wrong London, where real magic is reviled as the devil's work. Abandoned at birth, she has scratched out a living first as a pickpocket and then as a stage magician's assistant, dazzling audiences by secretly using her Changeling talents to perform impossible illusions. When she's dragged through a portal into the Witherward, Ilsa finally feels like she belongs.

But her new home is on the brink of civil war, and Ilsa is pulled into the fray. Beset by enemies on all sides, surrounded by supposed Changeling allies wearing faces that may not be their own, Ilsa must use all the tricks up her sleeve simply to stay alive.