In December 1840, Benjamin January is reunited with Arithmus, a Sudanese man of extraordinary mathematical ability whom he last saw in France fifteen years earlier. Their earlier acquaintance ended after British explorer Deverel Wishart died in terror at a supposedly haunted château and Arithmus disappeared.
January reopens the old death to clear his friend’s name, comparing memories, medical evidence and stories of supernatural vengeance. When the unresolved case creates new danger in New Orleans, he must discover whether the château concealed a murderer, a fraud or something that none of the survivors understood.