Operation – Annihilate!

The Enterprise follows a trail of mass insanity sweeping across inhabited worlds to the colony of Deneva, home to Kirk's brother Sam, his wife Aurelan and their son Peter. The landing party finds Sam dead and his family stricken. Aurelan dies after warning of creatures that have taken over the colonists' bodies. The cause is a swarm of parasites that attach themselves to the nervous system, cause terrible pain and control their hosts as part of a single hive mind. One attacks Spock, who struggles to master the agony. The team discovers that the creatures cannot survive intense light, and Spock volunteers to test the cure on himself, leaving him blind. They then learn that ultraviolet light alone would have worked. Satellites flood Deneva with ultraviolet light, destroying the parasites. Spock's sight returns, thanks to a Vulcan inner eyelid.