Here, let me just hit you over the head with this plot brick. The bit at the end with Joan's mother was obvious plot work, setting them up for Joan to want to stay with Sherlock after the weeks are up. For a show to be successful, you have to be able to balance the plot with the story and that did not happen in this episode.
The case was interesting enough and once again, Watson helped solve it. I enjoyed the focus on her and what her work means to her, though as stated above, it did end up heavy-handed.
Not a perfect episode, but not terrible, either.