This may not be the most exiting or incredible episode they've made, but in my opinion it remains as one of Star Trek and the Sci-Fi genre's most original episodes. Most ideas from retro Sci-Fi series especially including Star Trek has been reused several times, this one the other hand remains mostly as a one time triumph. This among the episodes that impressed me the most towards the end.<br /><br />Another thing I like with this episode is how it has accomplished to create such and exiting and captivating story with such few special affects. Now without criticizing the episode I must admit the effects are very dated, but then again what can you expect from a TV show from 1967? But still the creature ("Horta") in this episode is basically just a carpet with some coloured rubber on it. Yet you forget this after about 1 minute and you only start thinking about it as what it's supposed to be. Also the caves don't exactly look like rock, but again you forget it after a few minutes. This episode is a living proof on how good acting and a good story, can make you ignore the visual effects.<br /><br />The acting from the main cast is as usual great. This episode features the series second mind meld by Spock and is one of my favourites. So to say it simple Leonard Nimoy is definitely a scene stealer here, and his acting is excellent. Not that Kirk or Bones don't get their share of the episode but Spock is the most intriguing in this one. i like the fact that the episode is not about one specific character but evolves around the trio handling an alien problem. Also it's nice to see an episode who doesn't only happen on the Enterprise.<br /><br />Like most Star Trek episodes this episode tells us to have an open mind. I won't spoil the story, but evolves around what in the start seems like a typical monster story. It has killed several humans and therefore must be killed. But is that really all there is to it? I can assure you that the explanation in the end, will not disappoint you. This is still my favourite Star Trek TOS episode and i give it a 10/10.