Errand of Mercy

"Errand of Mercy" is the twenty-sixth episode of the first season of the American science fiction television series Star Trek. Written by Gene L. Coon and directed by John Newland, it was first broadcast on March 23, 1967. It was the first episode in which the Klingons appeared.

In the episode, with a war with the Klingons declared, Captain Kirk and his First Officer, Mr. Spock, attempt to sway the incomprehensibly placid population of a planet near the Klingon border to resist an invading military occupation.

The Klingon governor, Kor (played by John Colicos), would return in the Star Trek: The Animated Series episode "The Time Trap" voiced by James Doohan. He would return again decades later in the Star Trek: Deep Space Nine episodes "Blood Oath", "The Sword of Kahless" and "Once More Unto the Breach". In "The Sword of Kahless", Worf mentions Kor's encounter with Kirk.