Gemma Doyle, who runs the Cape Cod Sherlock Holmes museum, decides to travel to England to celebrate Holme's birthday by attending a convention. But the trip is immediately derailed when Gemma's father Henry recognises his brother-in-law Randolph who ran away years before, a troublemaker and forger of Old Masters paintings. Gemma is surprised when Randolph shows up at the Sherlock Holmes convention to sell a series of his sketches based on the Holmes canon. He isn't exactly popular with other attendees, and when he turns up dead after the banquet, Gemma sets out to find the killer.