Cloud William Roy Jensen Sirah Irene Kelley Wu Lloyd Kino Captain Ronald Tracey Morgan Woodward Lt. Galloway David L. Ross Lt. Leslie Eddie Paskey Dr. Carter Ed McReady

Arriving in orbit around Omega IV the Enterprise finds the U.S.S. Exeter. On being unable to raise any reply to hails, Kirk leads a boarding party to investigate. They discover that the ship is deserted but many Starfleet uniforms are lying around filled with a white crystalline substance. On examination, McCoy confirms that this is all that is left of the ship's crew. Spock accesses the computer and plays back the log which reveals that the Exeter crew contracted a strange disease and warns them that they, too, have now been infected and must go down to the surface in order to survive. Kirk has the party beamed down arriving in a primitive village to see a fur-clad man and woman about to be executed by people of Asian appearance. The captain of the Exeter, Ron Tracey, arrives on the scene and orders the couple to be locked up instead. He greets Kirk and explains that after beaming down to the planet he stayed with the villagers, called Kohms, when his crew went back to the ship. They succumbed to a disease that they took with them leaving him stranded on the planet. Something on the planet is keeping them healthy, but they cannot return to the Enterprise as they are all now infected. He also tells them that the two people they saw are called Yangs, a savage and warlike people bent on the destruction of the more peaceful Kohms. Lieutenant Galloway is injured during the next Yang attack and lies on the floor while Spock reports to Kirk that Tracey appears to be telling the truth as he has found empty phaser power packs among hundreds of dead Yang bodies. Kirk is about to contact the Enterprise when Tracey appears with some Kohms. Galloway draws his phaser but Tracey kills him. Taking the communicator he contacts the Enterprise saying that Kirk's party have been found unconscious. Later, Tracey tries to solicit McCoy's help in researching the effects of the planet on health as the inhabitants have extremely long life spans. The man with him, called Woo, is over four hundred years old and says his father is over a thousand. Kirk is imprisoned with the two Yangs they saw earlier, who try to kill him. Spock, in the next cell, manages to neck pinch the woman but the man is more wary. In trying to talk to him Kirk uses the word 'freedom' at which the Yang stops his attack, telling Kirk that 'freedom' is a 'worship word'. Between them they loosen the bars to the cell window but the Yangs knock Kirk out before making their escape. Kirk and Spock make their own escape, overpowering a Kohm guard and seeking out McCoy who has discovered that there is a natural immunising agent on the planet. It built up after a biological war and, because they have been eating and drinking the produce of the planet, are now immune from the disease which wiped out the Exeter crew. Tracey discovers them and orders Kirk to get the Enterprise to send down more phasers. While Kirk is explaining that there is no 'magic life potion' to be found on the planet and that fighting is useless, the Yangs burst in and capture them. Kirk and Spock surmise that the planet's development may be a parallel of Earth with the Kohms being 'Communists' and the Yangs 'Yanks' who have fought the biological war that earth managed to avoid. The Yang leader, Cloud William, who is the man Kirk helped to escape, brings out an ancient American flag and starts to recite vaguely familiar words. Tracey tries to convince the Yangs that Kirk and his friends are evil. Cloud William challenges Kirk to finish the recital of the 'sacred words' or Spock will be killed, but Kirk can't quite remember where he has heard them before. He suggests that good always triumphs over evil and the Yangs get him and Tracey to fight to the death to prove who is the evil one. During the fight, Spock uses mind control on a Yang female to make her activate a communicator. The Enterprise locates them and Sulu beams down with two crewmen to see Kirk defeat Tracey but refuse to kill him. The Yangs think Kirk is some kind of god and allow him access to their holy papers. Kirk looks at the text and realises that he has been hearing a very distorted version of the American constitution. He completes the speech and then explains to Cloud William that the words must be applied to all, Yangs and Kohms alike.