There is a planet, inhabited by species just like humans, but they live in very isolated clans or tribes. This clans contact each other only to make interclan marriages. This marriages arranged by some sort of local zodiac circle, because babies must be born under right stars or something like that.
Plot revolves around a girl from one of the clans, who refused to marry a guy from the other clan. The guy is furious and trying rape her, but realises that the baby from this action won't be "right". The girl is saved by some astronaut/researcher from another planet (possibly Earth), they had an affair, she gave birth to a child, that has no place in all the clans so she leaves it near the spaceship.