Sherlock investigates what he believes will be an open-and-shut case after Detective Bell is attacked. However, the case takes an unexpected turn when the suspected assailant is found dead. Meanwhile, Sherlock encourages Joan to learn self-defense.

Joan takes her first solo case: the disappearance of a woman who left her husband a tearful break-up video that mentioned a murder on a subway platform. Meanwhile, Sherlock is intrigued by the subway murder and decides to investigate.