The sign of the four

Several years have passed since "a study in scarlet". John Watson wrote the story of their adventure, thinking that Sherlock would be pleased, but the genius criticizes : there is useless romance, not only facts.

But Watson is mainly worried and upset about his companion drug habit : cocaine or morphine are supposed to stimulate Sherlock's bored mind, but how can he damage his extraordinary capacities ? 
Fortunately, a new case arrives with a charming visitor : Mary Morstan