Mycroft makes a surprise return and offers Joan a romantic relationship. When she tells Sherlock, he goes to Mycroft and tells him to back off. Joan reveals that she'd be interested in a relationship but firsts needs to move out of the Brownstone.
One of Sherlock's AA friends needs the duo's help when he sister goes missing. They uncover not only her body but another mans as well. They soon uncover the world of unmanned aeronautics and discover that a military contracted company kills them both to cover up a cover up.
Sherlock suspects that one of Mycroft's regulars is associated with French mafia, so Watson helps him investigate. In the end, Joan ends up getting kidnapped.