The Sign of the Four (1890), also called The Sign of Four, is the second novel featuring Sherlock Holmes written by Sir Arthur Conan Doyle. Doyle wrote four novels and 56 stories starring the fictional detective. (Wikipedia)
“A singular case,” remarked Holmes.
“I have not yet described to you the most singular part. About six years ago-to be exact, upon the 4th of May, 1882-an advertisement appeared in the Times asking for the address of Miss Mary Morstan and stating that it would be to her advantage to come forward. There was no name or address appended. I had at that time just entered the family of Mrs. Cecil Forrester in the capacity of governess. By her advice I published my address in the advertisement column. The same day there arrived through the post a small card-board box addressed to me, which I found to contain a very large and lustrous pearl. No word of writing was enclosed. Since then every year upon the same date there has always appeared a similar box, containing a similar pearl, without any clue as to the sender. They have been pronounced by an expert to be of a rare variety and of considerable value. You can see for yourselves that they are very handsome.” She opened a flat box as she spoke, and showed me six of the finest pearls that I had ever seen.
“Your statement is most interesting,” said Sherlock Holmes. “Has anything else occurred to you?”
“Yes, and no later than to-day. That is why I have come to you. This morning I received this letter, which you will perhaps read for yourself.”
“Thank you,” said Holmes. “The envelope too, please. Postmark, London, S.W. Date, July 7. Hum! Man’s thumb-mark on corner,-probably postman. Best quality paper. Envelopes at sixpence a packet. Particular man in his stationery. No address. ‘Be at the third pillar from the left outside the Lyceum Theatre to-night at seven o’clock. If you are distrustful, bring two friends. You are a wronged woman, and shall have justice. Do not bring police. If you do, all will be in vain. Your unknown friend.’ Well, really, this is a very pretty little mystery. What do you intend to do, Miss Morstan?”
“That is exactly what I want to ask you.”