In the Darwin Centre at London’s Natural History Museum, Billy Harrow, a cephalopod specialist, is conducting a tour whose climax is meant to be the Centre’s prize specimen of a rare Architeuthis dux—better known as the Giant Squid. But Billy’s tour takes an unexpected turn when the squid suddenly and impossibly vanishes into thin air. As Billy soon discovers, this is the precipitating act in a struggle to the death between mysterious but powerful forces in a London whose existence he has been blissfully ignorant of until now, a city whose denizens—human and otherwise—are adept in magic and murder.