The Enterprise receives an old style SOS signal and finds on arrival a planet that is virtually identical to Earth. Kirk, Spock, McCoy and Yeoman Rand beam down to the planet only to find that it is inhabited solely by children. Kirk befriends one of the older children, Miri, but they soon learn that experiments to prolong life killed all of the adults and that the children will also die when they reach puberty. They also learn that the children are in fact, very old. Soon, the landing party contracts the virus and has seven days to find a cure.