Harriet White is no stranger to odd correspondence. Ensconced in the basement of the Baker Street building society, her job is to reply to the mail they receive on behalf of Sherlock Holmes.

But while letters to the fictional sleuth may be plentiful, telegrams are rare, and so when she receives one describing the grave situation of celebrated author Philip St John, her interest is piqued. The writer describes St John as being consumed by terror, seeing ghostly apparitions on the fens, and only at ease in the company of his loyal wolfhound.

Before long, Harry finds herself in Cambridgeshire under the guise of being Holmes' assistant. The residents of Thurmwell Manor believe their master is cursed. Harry is sure there must be a logical explanation, but inside the echoey halls of the grand gothic house, her confidence in science and reason begin to crumble…

Can Harry solve the mystery before the fens claim their next victim?