Parasitic creatures spreading across the galaxy take over Kirk's brother's colony and threaten his crew.
The Enterprise arrives at Deneva, where the colonists are acting strangely, and finds that Kirk's brother and sister-in-law are dead, with their son in danger. Flying parasites control the minds of their hosts and cause terrible pain. Spock is infected, and the crew searches for a way to fight the creatures.
The episode ends the first season with a story of rising stakes, using the neural parasites as a threat. It is remembered for Spock overcoming pain through will, and for a clever solution using ultraviolet light. It also reveals more about Kirk's family.