Sherlock - I O U [ON HOLD UNTIL AFTER MY EXAMS]

*This is one short story, that will probably be in a series :)* Sherlock Holmes thought he’d seen the last of James Moriarty, when he pulled gun to his mouth on top of St. Bart’s Hospital. But little after a month later, Sherlock receives a message, and it can only be from one person. The question is, how?
This is a Sherlock fan fiction but you don't really need to have seen the films/tv show/ read the actual books, in order to understand this book :)