In an early part of the story, Billy the Kid hears a noise and calls out "¿Quien es?", Spanish for "Who is it?" Spending a lot of his time in Mexico, Billy the Kid spoke Spanish nearly as well as English. "¿Quien es?" was, in fact the last thing he ever said before being shot dead by Pat Garrett.
This episode takes place in Lincoln, New Mexico Territory on April 23, 1881 and in Arizona in 1968.
The title refers to the American western outlaw Billy the Kid also known as William H. Bonney (birth name Henry McCarty). After his participation in the Lincoln County, New Mexico range wars in 1877, he traveled through Texas gaining a reputation as a gambler, outlaw and gun slinger. He was eventually caught by Pat Garrett who later killed him after an escape from jail. Billy the Kid did not gain notoriety until after his death when his story was told in a sensationalist biography.