A surprise visit from Sherlock's brother Mycroft leads to discord between Holmes and Watson. Meanwhile, Sherlock and Joan investigate a missing person's case that takes them into the world of unmanned aeronautics.
Mycroft shows up in New York to ask Holmes and Watson to solve a case concerning his former fiancée. Mycroft and Joan's relationship is a source of contention with Sherlock.
When a young man is killed as he enters his apartment, Sherlock is called upon to consult. Watson will have to deal with Sherlock's reaction when she gets together with her ex-boyfriend.