Myrcoft returns to Elementary when he arrives in New York City and reveals he is opening a restaurant there. Not only brining his restaurant, he brings his ex fiance, Nigella whom Sherlock slept with, to ask if he and Watson could solve a case her for.
During the course of the investigation two things are revealed. The first being that Joan and Mycroft slept together when they were in London and not since talked about it. Secondly, that Nigella is still as rotten as she was when Mycroft was engaged to her.
Sherlock takes the news of Joan and Mycroft sleeping together quite well, despite the mocking and embarrassing questions he asks the both of them.
The investigation leads them through a web of horse breading and drug cartels. Sherlock proves the Nigella was coning many people out of thousands of dollars to keep her afloat after a nasty divorce. Myrcoft and Sherlock promise not to turn her in as long as she pays back the money she lied to people to get.
In the end, Sherlock and Mycroft make amends and become closer as brothers.