Sherlock: 10 Most Outrageous Fan Theories

4. Mary Works For Moriarty

As just mentioned, many fans believe there are still revelations to come about Mary's murky past - including the fact that she either did or still does work for Moriarty.

The idea goes that she is Sherlock's version of Colonel Moran, Moriarty's right-hand man in Doyle's story The Empty House - the tale that The Empty Hearse, the episode that introduced Mary, was based on. The theory is backed up by the similarity of Mary's maiden name - Morstan - to that of Moran. Some think Mary is running from this past while others believe she is still in Moriarty's employ, and that he planted her with John to destroy Sherlock's world from the inside.

This one was reignited by the idea apparently being hinted at in an unlikely source - the official Sherlock colouring book. On one page, Mary's wedding ring can be spotted in an illustration of the pool scene from The Great Game, in other words the place Sherlock first meets Moriarty. Pulling at straws or really on to something - you decide.