The Sign of the Four

In December 1878, Mary's father telegraphed her upon his safe return from India and requested her to meet him at the Langham Hotel in London. When Mary arrived at the hotel, she was told her father had gone out the previous night and not returned. Despite all efforts, no trace has ever been found of him. Mary contacted her father's only friend who was in the same regiment and had since retired to England, one Major John Sholto, but he denied knowing her father had returned. The second puzzle is that she has received six pearls in the mail from an anonymous benefactor, one per year since 1882 after answering an anonymous newspaper query inquiring for her. With the last pearl she received a letter remarking that she has been a wronged woman and asking for a meeting...