Sherlock Holmes. The Sign of Four. Level 3 + MP3 Audio Download: Arthur Doyle

Since her father disappeared, Miss Mary Morstan 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.