When Sherlock was younger he worked for M16 - as a spy. However, things happened and Sherlock is no longer on good terms with M16. He went into drugs for a short time but soon found work as a consulting detective for Scotland Yard (everything's canon from then out).
But, what happens when Sherlock's past comes back to haunt him?
Sherlock was a spy
But, what happens when Sherlock's past comes back to haunt him?