Derbyshire, England: Matlock and Matlock Bath are Edwardian towns clinging to the side of a steep and heavily forested gorge. The river Derwent snakes through the valley below. They are idyllic places in which to live.

But in the year 1893 something is lurking in the mists that roll down the valley. Hideous and relentless, it is murdering the inhabitants but masquerading the deaths in a fiendish way. The townspeople are divided. Are they victims of an irresistible force or is it just unfortunate happenstance?