Yonada was constructed to look like an asteroid and was sent on a computer-controlled course to a habitable colony world. The ship measured 320 kilometers in diameter. Chemically, it was a typical asteroid, however, it was self-propelled by atomic power and could correct for gravitational stresses. The engines left behind a trail of debris and hard radiation. The ship could defend itself with missiles.
In 2268, the USS Enterprise encountered the asteroid when it was fired upon by Yonada's automated defense systems.
The Enterprise crew discovered that Yonada was on a collision course with the populated planet Daran V. The inhabitants of Yonada did not know that they were on a ship. They called the ship's computer the Oracle and practised a sophisticated religion around it and its purpose to bring them to "the new world".
The Enterprise crew helped Natira, the leader of the Yonada colonists, to understand that their world was a ship, and corrected the course of the generational ship to its intended destination.
Natira was a Fabrini high priestess, leader of the inhabitants of Yonada. Dr. McCoy had been recently diagnosed with xenopolycythemia, a rare blood disease. Believing himself to not have long to live, he agreed to marry Natira. After Spock discovered a cure among the technological lore of the Fabrini, McCoy separated from her and returned to the USS Enterprise.