In case you forgot: A member of the Enterprise crew catches some condition that gives him mirrored contact lenses and incredible psychic powers. Kirk tries to figure out how deal with this threat, eventually killing him. A pretty lady psychiatrist dies in the process.
This episode is the second pilot; its original airdate as the third episode must have confused the heck out of its contemporary viewers, because the differences are significant. Spock is pissy and irritable, and his dynamic with Kirk just doesn't feel right. The chief medical officer is played by the bland and forgettable Paul Fix, and the costumes are some of the ugliest sweaters ever to grace a television screen. The "person with godlike powers" trope is going to get some more mileage before the series is over, unfortunately, and this isn't really the most compelling variation on the theme. The appeal of this episode, and why it isn't rated any lower, is in its value as the second Star Trek pilot. This is what the show almost was.