The Taunt / Death in Deadwood - S5.E14

A serial rapist seals his own fate when he sends a taunting letter to police, who use the DNA extracted from the saliva in the envelope's seal to track him down. And when a man is found dead in South Dakota, his head bashed in by a 50-pound rock, investigators go on a 20-year search for the killers--a man and woman who were part of a free-loving hippie group and are known only by the names "Outlaw" and "Inlaw."