With a rash of "pajama suicides" plaguing London, a cry goes out for noted private detective Sherlock Holmes, who is on a fishing vacation in Scotland, to investigate. Holmes quickly deduces that the deaths are actually murders, as no suicide notes have been left by any of the victims, but he tells his friend, Dr. John Watson, that he has decided to retire for heath reasons rather than take on the case. Soon thereafter, Holmes collapses and disappears, and the newspapers are filled with stories of the detective's death by drowning. In actuality, Holmes has faked his own death in order to go undercover to expose the murderous gang, which he suspects is led by a "female Moriarty." Holmes pretends to be an Indian officer named Raghni Singh who, like the previous victims, is a compulsive gambler. Adrea Spedding, the leader of the gang, falls for Holmes's bait and invites him to the Urban Casino, where, after losing all his money and contemplating suicide, he is told by Adrea that he can borrow money on his life insurance policy by making one of her friends his new beneficiary. The next day, however, Adrea discovers Holmes's disguise, and she orders her goons to place a poisonous spider in the air shaft of his apartment. Holmes foils her plot, however, and after killing one of Adrea's accomplices, the detective and Inspector Lestrade of Scotland Yard discover a wooden case and small footprints on the apartment's roof. Later, Holmes and Watson are visited at their Baker Street residency by Adrea and her mute "nephew," Larry. She pretends to be interested in hiring the detective to find her missing friend "Raghni," but as she leaves, Larry throws some candy wrappers into the fireplace, filling the flat with poisonous smoke. Holmes and Watson escape the fumes, however, then go to the home of Matthew Ordway, an eccentric specialist in insects. Holmes soon deduces that the man they meet there is an impostor named Radlik, but he makes his escape by releasing some of Ordway's deadly insects. Holmes and Watson later find the murdered Ordway, as well as the skeleton of a small individual. Holmes quickly realizes that Adrea has been using a pygmy to plant the poisonous spiders in the homes of her victims. Holmes and Watson then go to search for the pygmy at a local carnival, where the detective is soon captured by Adrea and her gang. Holmes is strapped behind a cutout of Adolf Hitler in a shooting gallery, but he escapes his binds just in time to avoid being shot to death by Watson and Norman Locke, Adrea's half-brother. Adrea and her gang are then arrested, and Holmes expresses his respect for the female's evil cunning.