The sign of four

The sign of four in YouTube.
What is the story about? A young lady named Mary Morstan comes to consult Holmes upon a case which she assures him is very unusual. Miss Morstan’s father disappeared some years ago under suspicious circumstances. He had obtained twelve months leave from his Indian regiment and sent word to her to meet him at his hotel. The hotel confirmed that he was staying there, but they informed Miss Morstan that he had gone out and not returned. A few years after this disappearance an advertisement appeared in a paper asking for Miss Morstan to reveal her address. Upon the advice of her employer she did so and that same day she received a valuable pearl in the post. Since then she has received one pearl a year for the last six years. Miss Morstan has now received a letter which urges her to meet with the sender that night. The anonymous sender permits her to bring two friends as long as they are not police.