Data accidently discovers a distress call from a little girl. He responds to her. However the species is not aware of aliens or space travel so the Prime Directive prevents the Enterprise to help.
It is one of the first explorations into the question of how much should man interfere in the natural order of things. In the end the enterprise does some magic and the planet is saved. But nobody is entirely sure if that was the right thing to do.
Later there are going to be better explorations of the prime directive.