There's a fine line between kill and cure. Edinburgh, 1849. Despite Edinburgh being at the forefront of modern medicine, hordes of patients are dying all across the city, with doctors finding their remedies powerless. But it is not just the deaths that dismay the esteemed Dr James Simpson. A whispering campaign seeks to blame him for the death of a patient in suspicious circumstances. Simpson's protégé Will Raven and former housemaid Sarah Fisher are determined to clear their patron's name. But with Raven battling against the dark side of his own nature, and Sarah endeavouring to expand her own medical knowledge beyond what society deems acceptable for a woman, the pair struggle to understand the cause of the deaths. Will and Sarah must unite and plunge into Edinburgh's deadliest streets to clear Simpson's name. But soon they discover that the true cause of these deaths has evaded suspicion purely because it is so unthinkable.