I can't remember the name of this movie. The girl is kidnapped by a boy about her age and his parents. They keep her in the basement in (if I can remember right) a sort of "garden." There is one other girl in the basement who dies later on. Eventually the girl earns the families trust and is allowed upstairs to sleep with the boy. The boy's dad tries to make the girl kill a woman that he kidnapped and tied up in the basement. I remember bits and pieces of the movie but can't remember the name and it has been driving me nuts!

Summer's Blood (http://www.imdb.com/title/tt1285010/)