Fabrini generation ship disguised as an asteroid, whose inhabitants did not know they lived inside a ship, found on a collision course with an inhabited world.
Yonada was a vast generation ship built by the Fabrini, whose sun had gone supernova some ten thousand years earlier. The vessel was hollowed out of an asteroid, with a habitable interior that its people believed to be the surface of their world, and it was governed by a computer known as the Oracle.
In For the World Is Hollow and I Have Touched the Sky, the Enterprise found Yonada on course to collide with the populated planet Daran V. Kirk, Spock and McCoy beamed inside and met the high priestess Natira, while the Oracle enforced obedience through devices implanted in its people. McCoy, suffering from the fatal disease xenopolycythemia, fell in love with Natira and chose to stay with her.
Kirk and Spock reached the control room and corrected the ship's course, and the Fabrini medical records on board provided a cure for McCoy. He returned to the Enterprise, while Natira remained to lead her people towards the promised world they were now on course to reach.