Stardate 3287.2: The Enterprise arrives at Deneva, a Federation colony where a so far unexplained mass insanity could have broken out, as on a couple of other planets before. The bridge crew witnesses how a Denevan heads directly for the system's sun, only to exclaim "I'm free!" briefly before his ship burns up. On the planet surface the landing party is attacked by inhabitants of the colony with clubs. Kirk's brother George, who is stationed on Deneva, is found dead. His wife Aurelan is taken to the Enterprise where she dies after fighting something in her body. Only Kirk's nephew Peter survives. The landing party finds unknown aliens in a building, one of which attacks and infects Spock. Back on the Enterprise Spock runs out of control but later manages to suppress the alien parasite successfully. McCoy devises a method to destroy the parasites on the planet with intense radiation of the kind that killed the one on the Denevan ship close to the sun. He exposes Spock to that light to test the procedure. Spock, who refuses to wear protective goggles because there are none on the planet either, loses his eyesight. Briefly later McCoy discovers that it would have been sufficient to limit the exposure of the parasites to UV radiation, so Spock's sacrifice was useless. UV satellites launched from the Enterprise clean the planet from the infection. In the meantime Spock's eyes have recovered. They were actually protected by an inner eyelid, a special Vulcan trait.

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