The Library of St. John the Beheaded contains the most dangerous books in all creation, so when some of them are stolen, who else should the Vatican call but Sherlock Holmes?
Immediately, one of the possible suspects seems more suspicious than others. He has no traceable background, refuses to give straight answers and hides behind a pseudonym. However, Holmes and his loyal friend Watson soon realise this suspect is also their greatest hope: war is brewing and an Old God is rising. To save humanity they need the Doctor as much as he needs them.