In an attempt to adapt Medusan technology to Federation use, and vice versa, the U.S.S. Enterprise picks up Medusan ambassador Kollos, instrument specialist Laurence Marvick and telepath Dr. Miranda Jones. The Medusans have a great beauty of character, but their physical appearance causes shock to the point of insanity in humanoids. When Dr. Jones turns down Marvick's proposal of marriage in favor of staying with Kollos, Marvick tries unsuccessfully to kill Kollos and is driven insane by a glimpse of the Medusan. He takes over the U.S.S. Enterprise engines and drives the ship out of the galaxy into an indeterminate region.

James Doohan (Lt. Cmdr. Montgomery Scott)
Diana Muldaur (Dr. Miranda Jones)
David Frankham (Larry Marvick)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Walter Koenig (Ensign Pavel Chekov)

• Diana Muldar makes a second appearance in the series, but as a different character. (Her first episode was "Return to Tomorrow", playing Dr. Anne Mulhall.)
  
• Larry Marvick was one of the original designers of the Enterprise.
  
• Dr. Jones can stand to look at the Medusans because she is blind.