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!!