The Secret Life of Professor Moriarty

When you're having detailed fantasies about commanding an army of elves in your office, you know you're bored. Moriarty's having one of those exceptionally dull days when, to liven things up, she walks in (I would have put that 'she' in italics, only you can't on summaries, you're just going to have to imagine)- a Time Lady, going by the name of Missy, looking for help.