A shuttle carrying Kirk, Spock, Bones, and Federation representative Nancy Hedford (Elinor Donahue) is pulled toward a small celestial body by a mysterious entity. Upon landing on the planetoid, the shuttle passengers discover Zefram Cochrane (Glenn Corbett), the inventor of warp drive, who had been presumed dead two centuries earlier. He had somehow been revitalized and kept in an eternal state of youth by the entity, known as the Companion (voice supplied by the frequently utilized Majel Barrett).
The episode is another analysis of life, discovery, and understanding in the tradition the classic-themed "The Devil in the Dark." The Companion and Cochrane have an interesting, affectionate relationship that might best be described as mutual co-dependence. Strangely, the episode's most interesting (and in some ways puzzling) notion is Cochrane's reaction when he learns the Companion is actually female. In fact, this reaction prompts us to rethink how love is defined, and even how gender might be defined. Since this lifeform is so utterly different from a human, how does the gender issue even apply? Is Cochrane or any human's love dependent upon the need for another human form?
In "Metamorphosis," Cochrane can't come to terms with the Companion's love for him until it merges into one with the body of the dying Nancy Hedford. "Metamorphosis" doesn't know all the answers, but it certainly poses some intelligent and probing questions.