Executive producer: Gene Roddenberry
Associate producers: Edward K. Milkis and Gregg Peters
Producer Fred Freiberger
Co-producer: Robert H. Justman
Story consultant: Arthur H. Singer

Writer: Joyce Muskat
Dir.: John Erman

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Gem: Kathryn Hays
Thann: Willard Sage
Lal: Alan Bergman

Stardate 5121.0

Two Federation researchers have disappeared in an area where the sun is just about to explode in a super nova. The Enterprise is looking for them. On their way to the last locations of the researchers, Kirk, Spock and McCoy suddenly find themselves in nearly empty rooms, with a young lady who cannot speak. She is the empath, and has the ability to heal a wound by touching the person and taking the pain on herself. McCoy calls her Gem. They find the missing scientists into big specimen jars, all dead. Three empty jars, labeled "Kirk", "McCoy" and "Spock" are ready for them. Two alien beings, Thann and Lal, tell the Enterprise officers that the scientists werte killed by their own fears and selfishness. The aliens openly admit to have abtucted either the researchers and the trio, because they need to make experiments on them. For that reason they torture Kirk brutally. When Kirk is back, heavily wounded, Gem takes the pain on her and heals him. Spock manages to knock out one of their captors with a nerve-pinch and take the device the alien used to control them and the environment. They find a way out and fly. In the open, they struggle in the storm (caused by the dying sun). They see Scott standing by a shuttle and waving at them, but it's just an illusion created by the aliens. Suddenly, they are back in the room they escaped from, any hope to free themselves gone. 

The aliens want to carry on with their experiments on a new specimen, either McCoy or Spock. They ask them to choose by themselves. Spock volunteers, but all the others rather prefer to have it on themselves than see one of their friends suffer. McCoy uses an injection to tranquilize Kirk, he's afraid his friend would try to go again himself. This makes things easy for Spock: He informs McCoy that it is now him who's in command and that he has decided to go himself. 

You made it easy for me, doctor. Now I am in command and choose who is to go: myself

Now McCoy needs to use another injection. Gem is impressed by the scene, she has found out that Spock is the most compassionate in spite of not showing it. She goes closer to him and, while Spock is distracted, McCoy approaches with his hypospray. As he faints, Spock declares that this is a misuse of his medical knowledge. When Thann and Lal come for the next, McCoy goes with them. He is brutally tortured. 

Awake, Kirk asks Spock why he had let him go and Spock dryly comments: "He convinced me the same way he's convinced you". When they get to see him, the empath feels with McCoy. He is dying. Spock takes McCoy's equipment and checks on him. He finds all vital organs heavily damaged and no chance to survive. When he softly takes McCoy's head in his hands, the doctors says: "Spock, you are a good doctor to your patients." The only way to save him is to ask Gem to do the same with McCoy as she had done with Kirk, though her own strength is fading. 

Kirk and Spok go to her, but they are kept in an energy field, a force which keeps them away from the woman. Now Lal and Thann explain to them that they have to decide which planet to save from the super nova. They can save only one. To find out whom to help they need to find out who has got compassion, because this is their priority. They have decided compassion and love are the most important components of beings and had hoped that Kirk's, Spock's and McCoy's actions (friendship, rather sacrificing oneself than a friend, compassion...) would teach Gem values she could take to her planet. The empath esitates at first, but then, out of her own will, she helps McCoy to survive, by healing him and taking his pain herself – she nearly dies. This is enough for Lal and Thann to decide that her race is the one they are going to save.