London, 1665. Hidden within a growing pile of corpses, one victim of the pestilence stands out: a young woman with a shorn head and pieces of twine delicately tied around each ankle.

Symon Patrick, rector of St. Paul’s Covent Garden, cannot say exactly why this corpse amongst the many in his churchyard should give him pause. Longing to do good, he joins a group of medical men who have gathered to find a cure for the plague, each man more peculiar and splenetic than the next. But there is another – unknown to The Society for the Prevention and Cure of Plague – who is performing his own terrible experiments upon unwilling plague-ridden subjects.

It is Penelope – Symon’s unwanted yet unremovable addition to his household – who may yet shed light on the matter. Far more than what she appears, she is already on the hunt. But the dark presence that enters the houses of the sick will not stop, and has no mercy…

Edinburgh, 1850. Despite 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.