Donald Hauser, a wealthy manager in the midst of a financial scandal, is found dead in his flat. Sherlock quickly suspects Chloe Butler, a friend of Watson's who prepared the victim's meals.
Joan discovers an unsolved murder connected to a rare prehistoric fossil when she reviews one of Sherlock's cold cases and Sherlock's responsibilities as a sobriety sponsor cross personal boundaries.
Shaw must stay close to a 10-year-old girl with highly developed surveillance skills after she is identified by the machine; Carter's mission could be exposed.