Arriving at the planet Deneva, home of Kirk's only brother Sam and his family, the U.S.S. Enterprise picks up a transmission from a Denevan pilot who has steered his craft into the sun to destroy some unknown menace. Beaming down to the planet, Kirk finds his brother dead and Sam's wife and son in a fatal condition. Before Aurelan dies, she informs the landing party that Deneva has been infested with large, amoeba-like aliens that attack humans and intertwine their tentacles with the body's nervous system, inflicting excruciating pain while forcing the victim to obey the entity.

DeForest Kelley (Dr. Leonard H. McCoy)
James Doohan (Lt. Cmdr. Montgomery Scott)
Joan Swift (Aurelan Kirk)
Maurishka Taliferro (Yeoman Zahra)
Majel Barrett (Nurse Christine Chapel)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Craig Hundley (Peter Kirk)
Fred Carson (First Denevan)
Jerry Catron (Second Denevan)

• The Deneva colony was colonized 100 years prior, a stop on the Earth Cargo Service run.
  
• The Enterprise has 14 science labs, one of which is shown for the first time in this episode.
  
• Bright light kills the creatures. McCoy used all light spectrums when testing the procedure on Spock, blinding him. Since Vulcans have an inner eyelid, Spock recovers from his blindness (naturally). 210 satellites are deployed around Deneva to bombard the planet with ultra-violet light.
  
• William Shatner (sporting a moustache) also played the dead body of Sam Kirk.
  
• Sam Kirk will appear on screen in live action in the premiere episode of Strange New Worlds.
  
• When they first encounter the parasites, Yeoman Zahra says, "They don't even look real." She was right about that. Regardless, this episode is still a classic.