This starts out well but ends badly. The basic premise is quite interesting (mass murder performed by an entity which inhabits unwitting bodies to carry out its need for fear), but the story may have been too big for one hour of episodic television.
What succeeds best is the early creation of tension. The murder scenes are models of efficiency for a prime-time TV show which cannot show blood or actual realistic murder. So the first murder happens off screen in a fog, and Scotty is plastered against a wall with terror. The second murder happens off screen with Scotty left holding the knife. And the third murder happens in the dark, with Scotty literally holding the body and the weapon. This goes a long way to creating the eery feeling that one of our heroes has gone terribly wrong. (Unfortunately, someone felt the need to provide a possible explanation for Scotty going off the deep end, so a couple of lines were inserted with regard to a woman-related accident in which Scotty got bonked on the head. It's lame and unneccesary, but mercifully brief.)
This setup, done at a very deliberate pace which allows the mystery and sense of dread to grow naturally, takes more than half the episode. By the time we get to the briefing room, there isn't enough time left to give the resolution its due. So we are given a courtroom scene which relies alternately on the computer's over-the-top accuracy, and Kirk's over-the-top sleuthing and guesswork. One can easily imagine this as a two-parter, or maybe even three if you also want to include the chase and goofiness which ends the episode.
Instead, we're spoon-fed the solution in a handful of lines of dialogue, and then dropped into a life-or-death scenario for which the viewer is little prepared. Then, having gotten themselves in that deep with a scant five minutes left in the show, they need a quick fix, and get it in the form of "tranquilizers for everybody!" It's a solution alright, but not very well conceived or executed. Sulu is just the first crewmember to go daffy laughy. And the ending is a forced giggle-fest which is wholly beneath the writing capabilities of Robert Bloch (who must have had to give in to Gene Coon or the Great Bird himself). It also is sufficiently dumb as to cause one to forget how riveting the first half was.
The guest stars are quite accomplished here in characters that are -- at first -- very carefully written. But not much is left of that when the mechanics of series television have intervened.
Rating: Middle-lower (5)