Edinburgh, 1849. Hordes of patients are dying all across the city, with doctors finding their remedies powerless. And a whispering campaign seeks to paint Dr James Simpson, pioneer of medical chloroform, as a murderer.

Determined to clear Simpson's name, his protege Will Raven and former housemaid Sarah Fisher must plunge into Edinburgh's deadliest streets and find out who or what is behind the deaths. Soon they discover that the cause of the deaths has evaded detection purely because it is so unthinkable.