Gemma Doyle, a transplanted Englishwoman, has returned to the quaint town of West London on Cape Cod to manage her Great Uncle Arthur's Sherlock Holmes Bookshop and Emporium. The shop, located at 222 Baker Street, specializes in the Holmes canon and pastiche, and is also the home of Moriarty the cat. When Gemma finds a rare and potentially valuable magazine containing the first Sherlock Homes story hidden in the bookshop, she and her friend Jayne (who runs the adjoining Mrs. Hudson's Tea Room) set off to find the owner, only to stumble upon a dead body.