Middle-class, professional family have a son and daughter and tried for a third, which was still-born.
The couple decide to adopt a third child and are charmed by a youngn Russian girl at an orphanage. They adopt her and although they know she's 'different' she seems to be almost the model child.
As time moves on, it becomes clear that she is not all she seems and has a personality disorder that means she's essentially unfeeling and uncaring. She manipulates the whole family to the point where they think the mother is being irrational and has unfair feelings against the new daughter.
The big 'twist' is that the orphan is not Russian and is not even a child, she is a 33 year old woman who was born with proportional dwarfism which causes her to have the appearance of a child. She had escaped from a mental home in Estonia and had a record of being 'adopted' and then making advances on the husband of the families, usually then going on to kill most, or all, of those families.