Joan takes on her first solo case: the disappearance of a woman who left her husband six months ago and hasn't been heard from since. The only thing she left behind was a tearful break-up video that mentioned a murder on a subway platform as a wake-up call - life is too short to stay with someone you no longer love. After interviewing the victim's husband, Joan thinks that he had something do to with her death. Proving it, of course, is another matter entirely. Meanwhile, Sherlock is intrigued by the murder on the subway platform and decides to investigate.