On the season 2 finale, Sherlock discovers that Myrcoft has been framed and promises him that he will clear his name. Just as Joan and Sherlock find the real mole and get a confession from his accomplice, Mycroft makes a deal with the NSA. The NSA takes care of the mole but Mycroft had to fake his death in order to protect Sherlock and Joan. 

Sherlock copes with his brother moving away and also with Joan moving out. After he pleads with Joan to stay and promises he will change, she still insists that she is moving out. Sherlock grabs a small bag of heroine and accepts a job from the MI6.