Sherlock and Joan assist the NYPD with investigating the death of a woman named Ella Jacobs, who was eight weeks pregnant despite a tubal ligation. They soon learn that her father is a notorious (now incarcerated) serial killer who murdered the same way that Ella was killed.
They realize Ella's husband could not be the father of her child due to a medical condition, and eventually learn that she was having an affair with the local realtor. The realtor is revealed to be the son of one of the original victims of the serial killer father, giving him motive.
Ella's husband beats him within an inch of his life before they can arrest him.
They eventually conclude that it was Ella's brother who killed her after she revealed her pregnancy; they had made a deal years before never to have any biological children in order to prevent their father's tainted legacy from being passed on, but Ella was going forward with her unexpected pregnancy anyway, so her brother killed her.
Sherlock is surprised and a little bit alarmed to discover Detective Bell at the brownstone when he comes home. Sherlock immediately assumes that Bell and Joan are sleeping together, a fact that Joan denies in annoyance. He later discovers that she is merely helping Bell study for the sergeant's exam.