Enterprise discovers a planet identical to Earth, with a very important exception: a few centuries earlier, a devasting plague struck. All of the adults were killed, but the children somehow survived - and still survive, hundreds of years later, feral and deeply distrustful of "grups". And there are monsters: insane, covered with scab-like plaques, they attack on sight. The disease is still active; the plaques appear on the landing party. If McCoy cannot discover a cure in the week they have left, all will die. Even Spock, now a carrier, cannot return to the ship.