Sherlock Holmes. The Sign of Four. Level 3: Arthur Doyle

Miss Mary Morstan has a strange story to tell.
Since her father disappeared, she has received a large pearl through the post on the same day, every year, for six years. Who is sending them? And what about her father`s paper with the words `The Sign of Four` written on it?
Sherlock Holmes alone can solve these mysteries.
Text adaptation by Jeremy Page.