Argylus was a class M planet located in the space of the galaxy's Alpha or Beta Quadrants. It was inhabited by an highly advanced humanoid civilization which was rated 17 on the industrial scale. They were so advanced that they were intellectually (but not emotionally or physically) mature by age three.
In 2259, Argylus was stricken by a plague which killed everyone over the age of thirteen. The children built their own civilization with a feudal-looking appearance, complete with castles, warlords, kings and robotic knights, but were unable to find a cure to the plague. One of the leaders, King Simon, began transmitting a distress call in the hopes of bringing to his world a starship capable of curing them. The other leader, Warlord Yago, had no hope that a cure would be found, and created games for his boys while they waited for death.
In 2266, the USS Enterprise, while en route to Zeta Centauri, received the transmission and arrived at Argylus, where James T. Kirk, Montgomery Scott, Hikaru Sulu and Christine Chapel beamed down to the surface. They were captured by robot chessmen and taken to Simon, who explained the situation to them. Leonard McCoy discovered a cure in the ship's databanks, which could only be found on Ominnus. While the Enterprise set out on the journey to that world, the landing party was captured by Yago, who first demanded that they hand over the cure, then imprisoned them in the dungeon prior to their execution. During an escape attempt, Kirk discovered that their rifles merely fired harmless light beams and that this was just one of Yago's games. Afterwards, the Enterprise returned, and the children were cured of the plague. (TOS comic: "Child's Play")