On an urgent medical emergency, Kirk, Spock and McCoy encounter an alien society that had once flourished on earth during the time of Plato. Since reaching their current planet, they've developed psychokinetic powers while also losing their bodies' ability to combat even the simplest infection, making the need for a physician newly apparent. When Dr. McCoy declines their invitation to stay, they begin to make sport with Kirk, Spock, Nurse Chapel and Lieutenant Uhura using their psychokinetic powers.