"For the World is Hollow and I Have Touched the Sky" is the eighth episode of the third season of the original science fiction television series, Star Trek. It was aired by NBC on November 8, 1968 and was written by Rik Vollaerts, and directed by Tony Leader (legal name: Anton Morris Leader).

In this episode, the crew of the Enterprise rush to stop an asteroid from colliding with a Federation world, but discover the asteroid is actually an inhabited generation ship.

Dr. McCoy, the Federation starship USS Enterprise's Chief Medical Officer, discovers he has a rare, incurable disease called xenopolycythemia (as in polycythemia) and has only one year to live.

The Enterprise is targeted by ballistic missiles, but they easily destroy the primitive weapons, tracing their point of origin to a large asteroid called Yonada. They find Yonada is on course to collide with the heavily-populated planet Daran V within a solar year, which would be catastrophic. They scan the asteroid and find it has a breathable internal atmosphere. Captain Kirk, First Officer Spock, and Dr. McCoy transport over to investigate. They discover that Yonada is populated by a highly religious society of humanoids that worship their Oracle. The High Priestess, Natira, orders the capture of the three and takes them to the Oracle. Kirk attempts to explain that they have come in peace but the Oracle knocks them unconscious with an electric shock.

When they wake, an old man approaches them, having heard of their arrival, and tells them that he has seen the outside of the asteroid, stating "...for the world is hollow and I have touched the sky". Suddenly he goes into shock and dies, and McCoy discovers a glowing device under the man's temple. They learn that all on Yonada have been implanted with such "instruments of obedience" under the Oracle's control. Reminding them again of the punishment for disobedience, Natira allows the three to explore the asteroid. McCoy stays with Natira, as the two are beginning to develop a relationship, while Kirk and Spock discover the asteroid is really a generation ship housing millions of beings of the Fabrini race. The ship was launched over 10,000 years prior to escape the destruction of their world by a supernova but the current residents are unaware of the nature of their world. Kirk and Spock, having secretly re-entered the Oracle room, watch as Natira privately prays to it about marrying McCoy. The Oracle agrees if he will accept an obedience device. It then viciously shocks Kirk and Spock again. Natira says she must execute them for their transgression but McCoy persuades her to relent. As Kirk and Spock prepare to beam back to the Enterprise McCoy insists on staying behind with Natira even if Starfleet has to destroy the generation ship to protect Daran V.

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