Errand of Mercy

TOS Season 1 Episode 26
Air Date: March 23rd, 1967

The first episode with Klingons. The Klingons declare war on the federation and the borders are quickly reinforced. The Enterprise is sent to the planet Organia which would provide a strategic advantage to whatever side holds it. The Enterprise arrives first and tries to convince the local population, a small civilization that appears to be in the medieval ages technology wise, to join the federation and they will be protected. The town elders refuse claiming they are safe.

Kirk tries to convince them that the Klingons are an agressive race and won't show mercy, but that doesn't sway the elders. Then the Klingons show up and the Enterprise has to retreat leaving Kirk and Spock on the planet. They pretend to be locals and the elders promise they would keep them safe. They accept the Klingons in their town and promise not to interfere.

Kirk eventually gets discovered and captured by the Klingons, but when the Klingon threatens to kill kirk, the elders step in and prevent it. Turns out they are a very advanced civilization that chose to not rely on their technology. They prevent the Klingons from killing Kirk and make them both agree to non violence while in their system. They even predict that one day the Klingons and the Federation will work together in peace.