On a lonely moor in northern England, the body of a young woman is discovered. In the south, a girl lies buried beneath a Saxon mound, while to the southeast, the ruins of a priory hide a human skull. Each is a sacrifice, a summons, and in this follow up to The Woman in the Woods, something in the darkness has heard the call. Making his 17th and most difficult investigation yet, Charlie Parker has also heard it, and from the forests of Maine to the deserts of the Mexican border, from the canals of Amsterdam to the streets of London, he will track those who serve these unspeakably sinister entities.