Gemma Doyle investigates after a poison dart sails through the window of the library at Suffolk Gardens House, where wealthy philanthropist and prominent Sherlockian David Masterson is hosting a lavish gathering.

"When a psychic fair arrives in West London, Gemma Doyle, owner of the Sherlock Holmes Bookshop and Emporium, wants nothing to do with it. But somehow, at the urging of Donald Morris, an enthusiastic Sherlockian, she finds herself talked into attending a séance, along with baker and best friend Jayne Wilson, store assistant, Ashleigh, and former pop star Bunny Leigh. But to her surprise, Gemma finds herself banned from the séance and shown the door....