Murphy was described as "crazy because someone killed his pa", and was kept locked up in Sheriff McLawdog's jail.
When a gang of robbers took over Pioneer Village, the leader, Franz, killed McLawdog and Vosky and Domino found Murphy in the jail and put him with the other hostages. Murphy stayed in character throughout the entire ordeal, like most of the other Pioneer Village employees. He acted antagonistic towards everyone, especially Pioneer Paul. When Franz sent his gang to investigate gunfire (Vosky shooting at Eric Cartman and Butters Stotch) leaving himself to guard the hostages, Stan Marsh, "in character" himself, told Murphy that Franz had killed his pa. With an in-character reason to attack Franz, Murphy lashes out and gives the police an opportunity to move in.
During the ensuing shootout, Murphy abandoned attacking Franz in favor of fist fighting with Pioneer Paul, even long after the police had killed most of the robbers and arrested Franz. He finally stopped when quitting time came, and broke character finally, removing his bald cap.
Murderin' Murphy is played by an unnamed actor with black hair. His costume consists of a poorly-placed baldcap that doesn't conceal all his natural hair or match his skin tone, fake red facial hair, a long-sleeved rawhide jacket with fur, a satchel, and a necklace with a rodent skull on it.