King Simon was a leader of Argylus in the 23rd century, despite being merely a child. Like all the children of his world, he was 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. King Simon began transmitting a distress call in the hopes of bringing to his world a starship capable of curing them. He managed to co-exist peacefully with Warlord Yago, the other leader of Argylus.
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 Simon's robot chessmen and taken to his castle, where he explained the situation to them, stating that they would die in five days if they failed to find the cure. While the Enterprise journeyed to Ominnus to retrieve the cure, Simon took the landing party to a medical research center to see if they could manufacture the cure there. Instead they were 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, including a grateful Simon. (TOS comic: "Child's Play")