The Ensigns of Command - S3-E2
Trivia: Due to a noisy aqueduct on the set, they had to go back and overdub virtually all of the dialogue in the pumping station scenes on the planet's surface. This is most apparent with the character of colony leader Gosheven, who was portrayed by guest actor Grainger Hines. After filming was completed, a schedule conflict prevented Hines from participating in the overdubbing sessions, so another actor rather mechanically voiced Gosheven's lines. The incongruous overdub was so obvious and so unflattering to Grainger Hines that he requested his name be dropped from the episode credits, and so it was.