Sherlock Holmes. The Sign of Four. Level 3, Оранжевый

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.