The very first episode of the original Trek series to air, though this was the 6th one filmed. The execs wanted to start things with a 'monster-of-the-week' mentality, but though the creature in this one is responsible for a number of crew deaths, it's more to be pitied than reviled by the end of the episode. It possesses startling chameleon powers, coupled with hypnotism & telepathic ability, enabling it to fool all the cast members during the story as it imitates several of them. Unfortunately, it craves salt, which it needs to survive, and human bodies fill the need very well. This is an example of sheer incompatibility between two races, which an angry Captain Kirk instinctively realizes in the 3rd act - he knows it's either his crew or the creature - there can be no half measures. We learn how intensely Kirk feels when he loses a crewman - check out the first killing here - and he has to go thru it a few more times in this episode.<br /><br />The Enterprise is on a routine check-up of 2 archaeologists on a planet with the ruins of an extinct civilization - but not all the inhabitants are extinct, as it turns out; there's one left. The storyline contains mystery and a chilling revelation. But, much of this episode emphasizes the routine: we never find out anything more about the dead civilization - it's probably one of many the Federation finds and explores; ship's and crew's routines are viewed during the creature's stalking, notably Yeoman Rand and Sulu; there's a casual banter between Kirk & McCoy which we all came to appreciate and look forward to in the next 3 years. Overall, this episode shows that future life, while with obvious differences in technology, will be largely the same for us humans. If there's a theme in this episode, it has to do with the inevitable extinction of a species, such as the American Buffalo, as if it's a rule of the universe. We can express sorrow for such a loss as part of the human equation, but, in the end, there's nothing we can do about it.<br /><br />However, the reason this episode probably gained some loyal followers of the Trek franchise right of the bat was probably not the monster. No, it must have been that easy interplay among the crew, especially the central trio - yes, including Spock, who right away starts insisting in this episode how he's immune to emotion but obviously enjoys a friendship with Kirk (Uhura even spells this out). And the human equation is delineated in McCoy's story in this one: he's reunited with his old girlfriend, so he's charmingly nervous as all hell, but in quick order, finds out she's been dead and her imitator must be killed, even though she's the last of her kind. Not only is this a personal tragedy for him, but he's also responsible for sealing the coffin on an entire species. The anguished look on his face towards the end is what makes this a memorable entry in the Trek mythos.