At first the interruption to the boredom of Sherlock Holmes seems to have little to do with crime. A governess, whose father vanished ten years ago and who has been receiving money each year, now needs an escort to meet her unknown benefactor. But before the night is over, murder has occurred.
As a dense yellow fog swirls through the streets of London, a deep melancholy has descended on Sherlock Holmes, who sits in a cocaine-induced haze at 221B Baker Street. His mood is only lifted by a visit from a beautiful but distressed young woman - Mary Morstan, whose father vanished ten years before. Four years later she began to receive an exquisite gift every year: a large, lustrous pearl. Now she has had an intriguing invitation to meet her unknown benefactor and urges Holmes and Watson to accompany her. And in the ensuing investigation - which involves a wronged woman, a stolen hoard of Indian treasure, a wooden-legged ruffian, a helpful dog and a love affair - even the jaded Holmes is moved to exclaim, 'Isn't it gorgeous!'