I loved this episode so much! Joan asked why Moriarty might have decided to be Holmes' nemesis! No-one ever asks that! And Joan is there because she accepts the danger and in fact thrives on it (and it's not like being a sober companion is a safe occupation nor, depending on her speciality, being a surgeon). I really liked that Gregson and Holmes both stated their concern (albeit in a somewhat patronising way) and Joan said no, you're wrong, I made my choice. Wonderful.