Gemma Doyle and her friends travel to London for a Sherlock Holmes convention--but will Gemma's father take the fall for a felonious forger's fatality? The 6th of January is Sherlock Holmes's birthday, and lucky for Gemma Doyle, January is also the slowest time of the year at both the Sherlock Holmes Bookshop and Emporium, and Mrs. Hudson's Tea Room. It's a good time for Gemma and her friends to travel to England for a Holmes Convention. For Gemma, the trip provides an opportunity to visit her parents, Jayne Wilson is excited about seeing all the sites London has to offer, and Ryan Ashburton just wants to spend some time with Gemma. But the trip is immediately derailed when Gemma's father Henry recognizes his brother-in-law Randolph Denhaugh, who disappeared more than thirty years ago on the night he stole a valuable painting from his own parents.
Henry, a retired detective with Scotland Yard, has been keeping tabs on the man's career as a forger of Old Masters and he warns Randy to stay away from his family. It's up to Gemma, with the help of her friends, to plunge into the "lowest and vilest alleys" of London to save her father from prison.