The Enterprise comes across a huge, antiquated vessel floating in space and carrying a colony of primitive human beings who have been lost in space.
While the colonists are in perfect health, they must deal with the culture shock of learning that a world exists outside of their spaceship.
Their initial reaction is fear, as they believe the crew of the Enterprise to be "demons."
And their fears are heightened with the belief that Captain Kirk and his crew are unaware of an outside force pulling them into a whirlpool of death.