This episode absolutely shocked me, it was a step up of the last episode (which I thought was still pretty good unlike the other viewers -watched it 3 times and every time for me at least, it got better) but this was was on another level. Moffat really carefully thought this through -in the first episode of Sherlock, it is mistakenly believed that John had a brother rather than a sister, and most of us as the audience followed Sherlock's footsteps and hung on tight to this 'brother' theory, however this time for Sherlock himself. The episode was set so meticulously, with the use of the Greek words and the mention of the 'east winds' and so forth which clearly show the immediate danger of Sherrinford. In The Six Thatchers, when Mycroft calls Sherrinford its not to her directly but rather a possible receptionist, which begs the question of whether or not she belongs to some type of institute -whether it be mental or anything really, which gives confinement it would explain her personality. Whatever it is though, she's been a mysterious character and has shocked (almost) everyone. Regardless of the episodes so far, 10/10 I think this episode was faultless and so carefully done.