The Sign of Four continues the partnership of Holmes and Watson as they are faced with an even more devious crime. A young governess named Mary Morstan comes to consult Holmes. Her father disappeared, presumed dead, nearly ten years ago. Four years after his death she began to receive strange gifts in the mail: once a year a large, lustrous pearl in an unmarked box. Now she has a received an unsigned letter calling her a wronged woman who deserves justice. Add to this a man killed in a locked room, a missing ancient treasure, a poison dart gun and a high speed boat chase on the Thames. This story has always struck me as one of most bizarre in the Holmes canon, but no less enjoyable for it.