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 is reunited with "M." a British criminal who appears to have followed Sherlock to New York. Meanwhile, Joan has reservations about leaving Sherlock and taking on a new client.