A flurry of unexplained suicides has recently occurred among the wealthy elite of London high society. Intrepid sleuth Sherlock Holmes (Basil Rathbone) believes the perpetrator is cunning seductress Adrea Spedding (Gale Sondergaard), a woman who perpetually haunts London's gambling halls in search of her unlucky prey. Now Holmes, with help from his steadfast assistant, Dr. Watson (Nigel Bruce), must draw Spedding out by playing the role of her next victim.