The Enterprise visits a planet long thought uninhabited, and finds globes that contain the consciousness of the last survivors of the planet, Sargon, Thalassa and Henoch. The three remaining beings wish to "possess" the bodies of willing Enterprise crew members, leaving the crew members' minds in the globes briefly as Sargon and his companions use the human bodies to construct android bodies for their minds. Kirk, Spock, and Dr. Mulhall agree to this procedure, but Henoch, occupying Spock's body, has other plans than building an android frame for himself. In the meantime, Sargon and Thalassa, in the bodies of Kirk and Mulhall, fall in love all over again. One way or another, though, the humans' bodies must be vacated since their metabolism is incapable of withstanding the levels of activity taken on by Sargon and the others.
