One of the "unknown people" is shot while atop a dam near Silsby, Texas. Superman catches him before he falls into water and takes him to the Silsby hospital. Superman, in his Clark Kent identity, assists a doctor who operates on the creature. The doctor discovers his patient has all the organs and internal body structure as humans. Meanwhile, the Luke Benson-led mob tries to kill the other visitor from the center of the Earth. That creature escapes, later bringing up more of his people along with a weapon. Superman races to defuse an explosive situation.
[Episode 25 and 26 together were originally the movie "Superman and the Mole-Men" (1951). Information from http://fandom.wikia.com/:
"Superman and the Mole Men is a 1951 black and white film which later aired as two episodes of the Adventures of Superman TV series called "The Unknown People."
It was the first theatrical feature film based on the Superman characters (although two live action Superman films had already been shown in cinemas, they appeared in a serial format). With an original screenplay by "Richard Fielding" (a pseudonym for Robert Maxwell & Whitney Ellsworth), it was shot in 12 days on a studio back lot. It served as a trial run for the syndicated TV series Adventures of Superman, with George Reeves and Phyllis Coates reprising their film roles as Superman and Lois Lane, respectively."]