Arena (Star Trek: The Original Series, Season 1, Episode 18)
Captain Kirk beams down to a desert planet to fight a space lizard. This episode marks the first sighting of the Gorn and the first (but certainly not last) encounter filmed at the Vasquez Rocks in northern Los Angeles. It's also one of the first times the crew of the Enterprise wrestles with the concept that maybe the Federation got something wrong -- a moral dilemma the show would revisit in every series.
Watch Star Trek: The Original Series on CBS All Access.