Star Trek: Season 1, Episode 26

1 - 26: Errand of Mercy

Kirk and Spock transport to the planet Organia, a pastoral realm of very low technology, which has the misfortune of being strategically important to the Klingons, who plan to use Organia as a forward base when they war on the Federation. Despite Kirk's best efforts, he cannot seem to impress upon the Organians that they are in dire peril and should ally with the Federation. Then the Klingons arrive, and being imposing martial law, and taking and executing hostages. Kirk works behind the scenes to interfere, but when the Organians betray him to the Klingons, he starts to wonder if they are worth protecting.