Banker Eddie Van Coon and reporter Brian Lukis are both shot dead in identical slayings, in rooms locked from the inside. Chinese museum employee Soo Lin tells Sherlock Holmes that, as a teenaged orphan in China, she ran drugs for the Black Lotus crime syndicate, for whom the two dead men also worked.

She too is then murdered, the killer being a human fly who can scale buildings. The gang mistake Watson for Holmes and capture him and his new girl-friend, requiring Holmes to come to the rescue.