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In an episode that comes the closest to creator Gene Roddenberry's original concept behind Star Trek (i.e., "Wagon Train to the stars"), the Enterprise is assigned to escort Kollos, the Medusan ambassador, back to his home planet in the company of his human companion, Dr. Miranda Jones (Diana Muldaur), a specially trained psychologist. In an irony-laced situation, Miranda is one of the most hauntingly beautiful women ever seen, while Kollos and the Medusans are a race of brilliant, sublime thinkers whose physical form is so hideous that a glimpse of them results in irreversible insanity for any human -- even Vulcans, with all of their intense mental discipline, can only look upon them with a protective visor -- and she is to spend the rest of her life with them, mind-linked to Kollos. Only Spock (Leonard Nimoy), a native of Vulcan, and Miranda, a born human telepath trained on Vulcan, can have any direct contact with Kollos. Spock discovers that Miranda is jealous (to the point of irrationality) of his Vulcan telepathic ability, his having any contact with Kollos, and knowing that he was the first choice for her assignment (only his devotion to Star Fleet prevented him from taking it). Her jealousy -- motivated in part by a secret she is hiding -- is so intense that even Kollos is repelled by it, jeopardizing the success of the mission. Adding to the tension of the situation is the presence of Lawrence Marvick (David Frankham), one of the Star Fleet engineers who designed the Enterprise and now assigned to build navigational equipment that will allow ships to make use of the Medusans' skills in this area. Once Miranda's mind-meld with Kollos is completed, Marvick falls in love with her and can't abide the notion that she plans to spend the rest of her life among the Medusans. At a social gathering onboard the Enterprise, she warns that someone aboard the ship is thinking of murder. Is her warning genuine or a reflection of her own unsettled feelings? Marvick finally snaps and tries to kill Kollos, with dire consequences for himself, the Enterprise, and Spock.