Approaching the planet Deneva, the Enterprise encounters a private spacecraft whose pilot is flying desperately into the sun, the apparent act of a lunatic. Spock reveals disturbing news: a pattern of mass insanity that has spread across the galaxy in a nearly straight line, the current end of which is Deneva. The landing party discovers the horrifying truth - and one of them becomes its victim. Now the crew must devise a cure before Spock dies, or is driven hopelessly insane.
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Complete story — includes the ending
The Enterprise arrives at Deneva - the home of Captain Kirk's brother, Sam, and his family - and discovers that the entire planet has been infested with large, amoeba -like aliens that have attacked and killed much of the human population. One of these aliens attaches itself to Spock, who volunteers to become a subject in Dr. McCoy's medical tests. McCoy and Kirk find a cure in time to save Spock and the remainder of the Denevan population.
Sources and content license
Source and revision history · Adapted from Wikipedia contributors, Star Trek: The Original Series season 1. HTML formatting and reference markers removed. · Content license