Edinburgh, 1850. People are dying all across the city, with doctors finding their remedies powerless. In this follow-up to The Way of All flesh, Dr. James Simpson is nonetheless blamed for the death of a patient, and so his protégé Will Raven and former housemaid Sarah Fisher are determined to clear his name. But as the trio plunges into the darker side of town, they discover that the true cause of these deaths has evaded suspicion purely because it is so unthinkable.