The Sign of the Four

At the outset of the novel Doctor Watson states that he has been watching Holmes takes cocaine three times a day for many months. Watson's reluctance to oppose Holmes is finally overcome and he confronts his friend about the potential ill effects of this prolonged usage. However, Holmes explains that the drug stimulates and clarifies his mind and that, without work to challenge his mental faculties, he becomes bored with day-to-day life. A young lady named Mary Morstan comes to consult Holmes upon a case which she assures him is very unusual. Her father, Captain Arthur Morstan, disappeared some years previously under suspicious circumstances.