CBS

Season 2: Episode 4 - Poison Pen

Poison Pen: Investigating the murder of man by nitroglycerine poisoning, Sherlock discovers the family’s nanny is a woman from his past who he knows killed her own father with nitroglycerine. Sherlock tries to prove she was framed for this crime. Video evidence is discovered that points to the elder son, who was the victim of sexual abuse by his father. Sherlock, Joan, and Gregson are about to hear his confession when the nanny confesses to save the boy from jail.