While investigating an explosion that kills two people, Sherlock discovers a crime that’s been lying dormant for years. Meanwhile, Watson encourages Sherlock to finally pick a sponsor.

Sherlock and Joan try to prevent the largest cash vault in the U.S. from being robbed during a deadly blizzard. Meanwhile, Holmes gives shelter to Ms. Hudson, a transgender woman who is recovering from her latest breakup.

When a financial consulting company CEO is poisoned to death, a woman from Sherlock's past who emerges as a possible suspect.

When a body is found in a barrel, Holmes and Watson launch an investigation into the mafia’s ties to the murder. Meanwhile, Sherlock attempts to make amends with Detective Bell.