One thing I noticed about Sherlock is that the second episode really doesn't have anything to do with the first and last episodes. I'm guessing that since it was "Created by Moffat and Gatiss", and episode one was by Moffat and episode three was by Gatiss, they must have just gone "We can't just have two episodes, oy you, write us another one to stick in between here."

Great series though. I've been a fan of Moffat for years and this show didn't disappoint (he also did Jekyll, which is just as good, if a bit campier at times).