Newly arrived in 1892 New England, Abigail Rook becomes assistant to R. F. Jackaby, an investigator of the unexplained with the ability to see supernatural beings, and she helps him delve into a case of serial murder which, Jackaby is convinced, is due to a nonhuman creature.

Detective Somerset has seven days before he can retire from work and escape from the city. Detective Mills is starting his first week on the job. This story is about seven days and seven shocking murders. The serial killer has one thing on his mind-- seven deadly sins.