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.

Holmes and Watson investigate a murder that initially looks like a road rage incident, but takes a new direction when they learn the victim was flash-frozen at the time of death. Also, when Watson’s mother finds out Joan’s brother is cheating on his wife, she puts Joan in the awkward position of confronting him about the affair.

Holmes and Watson race to help Detective Bell when he becomes a victim of a bio-terrorism attack at the precinct and the station is quarantined. During the lockdown, Captain Gregson and Detective Bell begin to suspect they are trapped inside with the perpetrator. Also, Watson faces the difficult task of helping her mother, Mary, accept that her advancing Alzheimer’s disease requires her to receive more medical care.