A novella set in the world of the "Rivers of London" series, set in the US and between the events of "The Hanging Tree" and "Lies Sleeping". When retired FBI Agent Patrick Henderson calls in an ‘X-Ray Sierra India’ incident, the operator doesn’t understand. He tells them to pass it up the chain till someone does. That person is FBI Special Agent Kimberley Reynolds. Leaving Quantico for snowbound Northern Wisconsin, she finds that a tornado has flattened half the town – and there’s no sign of Henderson. Things soon go from weird to worse, as neighbours report unsettling sightings, key evidence goes missing, and the snow keeps rising – cutting off the town, with no way in or out. Something terrible is awakening. As the clues lead to the coldest of cold cases – a cursed expedition into the frozen wilderness – Reynolds follows a trail from the start of the American nightmare, to the horror that still lives on today…