I think this episode is the best of the third season.

This episode creeped me out as a kid. That ghost was scary! All I remember of it was the part where the ghost met his end. Watching it again, I got about half way through before I realized it was the ghost episode.The ID thing was well done. The possessed Dr. Smith was truly menacing and scary! For the first time in the whole series it looked like Will was genuinely in danger.The power core was cool! I wonder what was in those boxes. Plutonium fuel pellets or something?How come the robot has sensors that can tell him about the planet, but the ship doesn't? In fact, why didn't they equip the ship with sensors, artificial intelligence, and a voice interface?

I actually agree with the other reviewers on this one. This, along with The Time Merchant, are the best episodes from Season Three. Weirdly presented and written, this has many solid elements and a nice sense of mystery. Only flaw: The Bed-sheeted Ghost creature is a real lax of imagination on the writer's part, but it doesnt damage the fun.
The evil, ID'ed Dr. Smith is almost alarmingnly frighting and its all given an underlying layer of Star Trek foolishly elevated to a much more palatable moral. An odd-ball, but super episode. Bravo!!

Wow! Great costume work, there. They spared no expense on that . . . Bed sheet.Despite that bid of sarcasm, I guess this episode was actually ok. The idea of the cast vanishing into another dimension was different, and the idea of William's ID being the creature was good too - and Dr. Smith as the puppet of the ID creature was actually creepier than his usual performance. I think it may have been his best work on the show.

While the science content of the episode could be written better by my third grade daughter, even she was delighted by the creepy monster and I enjoyed its unusual motivations. This is despite the fact the monster is no more than a man in a bed sheet. However, the wonderful vocal performance provided for the creature by Ronald Gans makes you forget about that - just as his performance does in the ST:TNG episode "Skin of Evil" where he provided the voice for Armus, another evil, sheet-like monster.Dr. Smith's antics are minimized, Jonathan Harris gets a chance to play a nasty, and the whole cast gets screen time instead of it just being Will and the Robot. Watch it with your kids and see in their eyes the joy and mystery you had when this first aired.

While the science content of the episode could be written better by my third grade daughter, even she was delighted by the creepy monster and I enjoyed its unusual motivations. This is despite the fact the monster is no more than a man in a bed sheet. However, the wonderful vocal performance provided for the creature by Ronald Gans makes you forget about that - just as his performance does in the ST:TNG episode "Skin of Evil" where he provided the voice for Armus, another evil, sheet-like monster.Dr. Smith's antics are minimized, Jonathan Harris gets a chance to play a nasty, and the whole cast gets screen time instead of it just being Will and the Robot. Watch it with your kids and see in their eyes the joy and mystery you had when this first aired.

While the science content of the episode could be written better by my third grade daughter, even she was delighted by the creepy monster and I enjoyed its unusual motivations. This is despite the fact the monster is no more than a man in a bed sheet. However, the wonderful vocal performance provided for the creature by Ronald Gans makes you forget about that - just as his performance does in the ST:TNG episode "Skin of Evil" where he provided the voice for Armus, another evil, sheet-like monster.Dr. Smith's antics are minimized, Jonathan Harris get a chance to play a nasty, and the whole cast gets screen time instead of it just being Will and the Robot. Watch it with your kids and see in their eyes the joy and mystery you had when this first aired.