In 50,000 acres of remote Scottish mountains and moor sits Dorcha Hall: a former hunting lodge nestling amongst ancient forests, on the shores of the enigmatic 'Black Loch'. At night, lights glow from within, smoke curls from its chimneys, and in the forgiving murk of the gloaming, it looks magical. In daylight, its mouldering plasterwork and failing plumbing present a less romantic reality.

Dr Ava Dickens has come to Dorcha to visit her godson, Alastair – a young man whose parents' sudden death has taken him from a quiet life in academia to owning Loch Dorcha. He and his very new wife are convinced the only way to make the estate work is to run the Hall as a hotel. But Ava is a rigorous scientist, her specialty animal behaviour, and she is by no means certain that the evidence is indicative of a successful outcome.

When one of the hotel guests is found dead in his bed, having gulped down poisonous insect repellant rather than the whisky sitting next to it, Ava's knowledge of the female Culicoides impunctatus – aka the Highland midge – leads her to doubt that it was an accident. And worse is to come…