Sherlock is called in to investigate when a supposedly uncrackable bank vault called “The Leviathan” is breached. Meanwhile, Joan is pleasantly surprised when Sherlock meets her family and defends her choice to become a sober companion.

Sherlock resumes his hunt for Moriarty when Sebastian Moran reveals that a man who was thought to be dead from a heart attack was actually on the list of intended victims from Moriarty.