'Elementary' Season 2 finale: Oh Sherlock, no ...
It turns out the mole inside MI6 who has set Mycroft up for treason and the murder of Arthur West is none other than Tim Sherrington, Mycroft's handler since he became an asset for the British intelligence agency. It's looking like Holmes and Watson have figured out enough to put Sherrington away (by getting someone who hired Sherrington to give evidence against him), but Sherrington threatens the only people Mycroft cares about -- Joan and Sherlock -- so instead, Mycroft goes to the NSA, uses the information he has gleaned from his work with MI6 to get the NSA to murder Sherrington and agrees to fake his death and disappear forever.
It breaks Joan's heart for sure and it would seem it breaks Sherlock's heart as well -- though some of his heartache also comes from the fact that Joan is still moving out of the brownstone.
All of this causes Sherlock to get the bag of heroin out of the book where it has been hidden (nooooo!) and then go agree to work for MI6.
What does this mean for Season 3? We don't know. What will the show be without Holmes and Watson working with Gregson and Bell at the NYPD? And don't even get us started on how disappointed we'll be if Sherlock turns back to drugs (not disappointed in the show, it's not a stretch of a plotline, but disappointed in the character).
Mycroft: "Do you really think --"
Sherlock: "That I can convince them I dislike you enough to help them bring you to justice? I shall dig deep."
Sherlock to Watson: "When I look back on the last 18 months, I often categorize it as a kind of 'grand experiment,' the results of which have demonstrated to me, much to my surprise, that I'm capable of change. So I will change. For you. For the sake of our partnership, for the sake of our work, stay."