In London a dense, yellow fog swirls through the streets.
At 221B Baker Street, Sherlock Holmes sits in a cocaine-induced haze, deeply melancholic.
His mood lifts when a young woman, Mary Morstan, visits regarding her father who had vanished ten years before.
Four years later she began receiving a large, lustrous pearl every year.
Now she has had an invitation to meet her unknown benefactor.
She asks Holmes and Watson to accompany her, little knowing that others stalk London's fog-enshrouded streets: a one-legged ruffian with revenge on his mind, and his companion who places no value on human life...