Miranda Jones, a telepath, is jealous of Spock's greater abilities in
forming a mind-link with Kolos, an alien so ugly that the very sight of
him can drive a man insane.
Kolus is in a small box (great way to save on the F/X budget). Spock
forgets to wear his visor (with some help of Miranda), causing him to go
crazy and take the Enterprise out of the galaxy.
Spock deduces that Miranda is blind, and McCoy realizes that her dress is
giving her information on her surroundings (she can judge distances much
more accurately than unaided humans).
FACTS
Kolos is a Medusan.
First use of prosthetic visual device (Miranda's dress sensor array).
Spock wears the IDIC out of respect for Dr. Jones. IDIC = Infinite
Diversity in Infinite Combinations.
Kirk doesn't wear his protective visor at the end of the episode, but
since he's superhuman anyway, apparently he isn't effected.
Spock mind melds with the Medusan.
OPINIONS
Why didn't McCoy know about this dress thing? Was it Federation science or
alien?