Watson is understandably worried about her client, whom she's about to finish with and move on to another recovering addict. She's reluctant now to end her time with him, but it's not only her concern for him and how close he is to the frayed end of his emotional rope. She is also reluctant to leave the chase, the puzzles and the excitement she gets from her affiliation with Sherlock.
Jonny Lee Miller is superb in "M." It is a controlled, but emotional performance. He is cold and calculating, methodical in intent. His eyes and tightly coiled body language reflect not only the ruthlessness with which he intends to deal with "M," but also the deep emotions this encounter has brought to the surface. There is a sense that Holmes is somewhere else in his head entirely, frightening to Joan and to us.
Miller is also great in conveying to us his feelings about Watson's departure. He says little until the emotional reveal to her at the end of the episode, but it is clear that his reticence says more than words can. Holmes' admission at the end that he truly values what Watson has done for him is a great payoff coming at the end of an emotionally exhausting night for Sherlock.
It is both Watson's worry about Sherlock's emotional state and her desire to keep working with him that compels her to phone his father, requesting to stay on as his recovery companion. Although he declines to continue her services, she chooses to lie to Holmes, telling him that his father will continue to pay her. I believe she does that to protect her own conflicted feelings as well as not reveal the extent of her worry about Holmes at this point. Liu also does a great job at conveying Watson's emotions while trying to keep them in check.
It is a great place to leave the story as the series takes a short break before returning with new episodes at the end of the month. In case you missed last night's episode, you can watch it on the Elementary official site. Elementary airs Thursday nights at 10:00 p.m. on CBS.