The Hamiltons are a family of vampires.
Francis (Cory Knauf) doesn't like the fact that they're kidnapping people and keeping them in their basement to slowly drain them to death and tries to free one of them. David (Samuel Child) tries to explain to him that he was once like Francis, but once the "Change" happens, he'll be one of the family. Francis does change while trying to help the girl escape and ends up killing her and feeding off of her.
By now things have gotten too dangerous for the family to stay in the house so they're going to have to leave. They need to get Lenny who's been locked in a room in the basement during the whole movie and who is so vicious he even bit up Francis' hand at one point.
Lenny turns out to be their younger brother and is locked up because vampire children can't control their viciousness and are like wild animals until they get older.
Darlene lets him out and he's fine.
The final shot is of the family moving into a new neighborhood.