Joan discovers an unsolved murder connected to a rare prehistoric fossil when she reviews one of Sherlock's cold cases; Sherlock's responsibilities as a sobriety sponsor cross personal boundaries.

When the leader of a local street gang abducts Holmes to compel him to find the person responsible for a hit on his men, Holmes and Watson get caught in the middle of an international hunt to recover a centuries old, priceless artifact. Also, Watson debates whether a personal favor Shinwell asks of her is outside her comfort zone.