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: Rick Vollaerts
Dir.: Tony Leader

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
Lieutenant Uhura: Nichelle Nichols
Nurse Chapel: Majel Barret
Natira: Kate Woodville

Stardate Stardate 5476.3

A planet is on collision curse with another one. Heading for the oncoming planet, the Enterprise discovers that it is a spaceship disguised as a planet. Having completed the medical checkup Dr. McCoy has found out that all are o. k. except one who's terminally ill: The chief medical doctor of the Enterprise. He wants to stay in duty as long as he can. On the planet Kirk, Spock and McCoy are "greeted" by people attacking them. Overpowered by the many they are taken inside the planet/ship and Natira introduces herself to them, she is the high priestess and introduces them to the "oracle" which determines the fate and rules of the ship. 

No one seems to be aware that they are on a ship, they call it their world. The oracle accepts Kirk, Spock and McCoy's introduction to come as friends, but also lets them experience what it means to be an enemy: They feel extreme pain which renders them unconscious. McCoy wakes up last in their room and Kirk tells Spock about his condition. They are visited by an old man who shares with them that he has been outside (which is forbidden) and that he has seen the stars. While talking he feels frequently pain in his head. When he says that their world is hollow and he has touched the sky, he dies of the pain emerging from the side of is head. Natira comes and takes care of him. 

Natira and others bring refreshments for their guests and good news: The people of Yonada, as their world is called, know now that they are here. Spock's interest in the culture is gladly accepted and they are allowed to go where they like. While Spock and Kirk try to find out about the oracle, McCoy stays with Natira and both fall in love. He shares about his illness and he is willing to spend the rest of his life's expectancy (12 months) on Yonada. Spock had watched Natira to open the doors for the oracle. Inside they try to find out about the origin of the ship. Spock thinks that they are Fabrini, coming from a system which ceased to exist. Suddenly Natira comes in and kneels down before the oracle. She asks for permission to marry McCoy. The oracle asks whether he also wants to marry her. Kirk and Spock hear that McCoy has wished to stay on Yonada with her. 

The oracle gives it's consent, but makes clear that McCoy has to get the implant which makes him one of the Yonadans. Before Natira leaves, the oracle finds Kirk and Spock and paralyses them with pain. They have to leave the world. McCoy is sure: He wants to stay with Natira. After he got the implant in his head Natira shows him how to open the holy place where a book is kept. 

Doctor, your choice is illogical
Do you really think so, Mr. Spock?

Back on board Kirk and Spock get a message from McCoy. He tries to tell them where the oracle can be opened and where a boo can be found. The conversation is ended by the oracle, which nearly kills McCoy. Hearing McCoy slowly fade Spock and Kirk go back, find McCoy and remove the implant. Natira is devastated. She loves McCoy and knows that he is not a part of their people any more. Half awake, McCoy tells them where the book is. Though the oracle tries to defend itself by tremendously increasing the heat, they find the book and Spock deciphers how to open the way to the steering computer. 

Inside Spock finds out that one engine does not work any more, this explaining the incompetence of the highly advanced machine to avoid collisions. Spock corrects the device and copies the data from the medical memory bank of the computer. By that knowledge McCoy can be healed. He goes back with them on board and the collision with the planet is avoided. Kirk promises McCoy that he will be there when the people soon reach their destination.