The Enterprise arrives to pick up Ambassador Kollos, a representative of the Medusan race. The Medusans are a non-physical race reputed to be so hideous in appearance as to drive any human insane who sees them. Vulcans are partially immune. Kollos s is traveling with engineer Larry Marvick and Federation liaison Dr. Miranda Jones. Spock beams Kollos and Jones aboard: Kollos is contained within a metal box. Spock takes Kollos and Jones to the ambassador's quarters and the ship leaves orbit for their rendezvous. Spock asks to exchange greetings with Kollos and after Jones approves it, they open the box to reveal a glowing display of lights. Jones is a telepath who received training on Vulcan but hasn't yet achieved a true mind-link with Kollos. After Spock leaves, Jones demands of Kollos what Spock saw when he the ambassador. At a reception, Kirk wonders how Miranda is able to look at the Ambassador and she cites her mental discipline, which she learned as a born telepath when she had to develop the need to shut out other people's thoughts and emotions. Miranda notes Spock's IDIC symbol and wonders if he is mocking her, but Spock claims he is wearing it as recognition of her talents. The intent for Starfleet is to learn a way to have the Medusans act as navigators for starships, with Marvick doing the design work. They share a toast but then Miranda picks up a stray thought of murder. She dismisses it and makes her leave, while Scotty tries to get Marvick off for a drink, as Marvick is one of the designers of the Enterprise. Marvick pleads fatigue and leads, while Spock says that he truly intended to honor Miranda. Marvick goes to see Miranda in her quarters and insists she not go with Kollos. He's been pursuing her romantically for some time but she isn't interested. As she dismisses him she gets an image from his mind of murder, but isn't sure if he wants to kill her or Kollos. Marvick storms out of her quarters then goes to Kollos' room. The ambassador emerges from his box as Marvick tries to use his phaser, and Marvick sees him and goes insane. Miranda senses a telepathic disturbance and goes to investigate, while Marvick stumbles into the corridor and goes down to Engineering. Miranda recovers the phaser and seals the box, then goes outside to inform Kirk and the security team what is going on. Scotty turns the controls over to Marvick just as the red alert sounds. Marvick struggles with the crew and manages to overcome them, then takes the controls and throws the Enterprise into maximum velocity. As Spock and Chekov try to override, Kirk, McCoy, and Miranda go to Engineering and subdue Marvick but it's too late: the Enterprise has left the boundaries of the galaxy. Miranda tries to calm him but Marvick attacks her and then falls dead of the strain. The Enterprise finds itself in an unknown space-time continuum. The engines are fine but they have no navigational referents. Spock suggests he telepathically merge with Kollos, an expert navigator, and warns that their personalities might merge. They assume Miranda will oppose the joining and Spock suggests they distract her. Kirk takes her to the arboretum where they admire the flowers. She insists on shutting down her emotions as she finds them uncomfortable, and Kirk wonders what she will do for love. Miranda wonders if Kollos is too ugly or too beautiful to bear. Kirk tries to kiss her but Miranda puts him off. She then telepathically picks up the thought of Spock's plan and goes to Kollos' quarters, but Spock notes that she is blind, and uses a sensor web to navigate. Miranda insists she can navigate the ship but they suggest she take it up with Kollos. She mind-links with him and he tells her that she must let Spock undertake the operation. Kollos is taken to the bridge and placed behind a protective shield, and Spock puts the visor on and merges telepathically with the ambassador. The sharing results in a more emotional Spock-Kollos. He apologizes for disturbing the crew and then navigates them back to their original position in the galaxy. Spock appears to be overwhelmed by Kollos' personality and Kirk suggests they break the meld. Spock returns behind the screen, but forgets his visor. The Vulcan screams out in agony then emerges and attacks the crew until Kirk stuns him. Only Miranda can bring Spock out of his insanity but they're skeptical as to whether she wants to help a rival. Kirk goes to confront her and when she says that she can't help him, he accuses her of leaving Spock to die and possibly forcing him telepathically to forget the visor. He accuses her of jealousness and storms out, and Miranda attempts to bring Spock out of his insanity. A few minutes later a restored Spock staggers into the medical office. Later, Miranda prepares to depart and reveals she is now one with Kollos, having set aside her jealousy and hatred. She thanks Kirk and acknowledges the values of IDIC to Spock, then leaves with the ambassador.