Kirk and the gang encounter a group of children, living on a planet where the adults seem to have disappeared a long time ago. Upon arrival, they find a man dying in agony and don't understand the how he arrived at his condition. Investigating further, they come upon what appear to be children who act like six year olds, even though they are actually 300 years old. Dr. McCoy comes to the conclusion that there is something about the planet that causes a horrible demise when puberty is reached. Kirk tries to negotiate with the kids because they think the adults are responsible. Kirk meets Miri (Kim Darby) who develops a crush on him. Unfortunately for her, she is beginning to show symptoms similar to the dying man. She has another ally, a young man played by the very strange Michael J. Pollard who was the driver in the "Bonnie and Clyde." The kids just won't trust the crew and this presents a problem for Kirk, who begins to turn on the charm. The kids are really freaky and play violent games and won't listen to reason. Why after 300 years they have not evolved any further than they have is a real mystery. Will the condition that is affecting Miri be circumvented? That is the question.