In Kindred, Dana, a present day descendant of a slave and her white owner, finds herself traveling back in time involuntarily. She begins to understand that she does this when the life of one of her ancestors is in danger; when she saves them, she ensures her own present-day existance. Butler uses this plotline to show the horrors of slavery. One of Dana's ancestors is owned by the other. When Dana is in the past, as a black woman she, too, is treated as a slave. She has to save people she does not like and endure situations that are horrible, in order to allow her own future self to be born. Butler shows various horror of slavery--rape, physical brutality, separation of family members--while writing an interesting time travel novel. She was one of my favorite SF authors, and died far too young.