It started out interesting, then went off the rails. Ending spoilers

The beginning was intriguing, and very soon - not far into the movie, you start thinking that there is something off about the parents. After the girl begins talking to the kid,I thought maybe these parents abducted the girl from the neighborhood to replace the daughter they accidentally killed and maybe that would have been more interesting.

So the parents had Sarah, the girl, and she was what ... deformed? A monster, or some kind of supernatural being? Or did they lock her away for some other reason and then she developed into some monster?

I liked the general vibe of this movie from early on, but so much happened that seemed ridiculous and the very end was confusing, she was threatening that she could not be held in the cage, and she would haunt the kid forever. And that's where the movie ended... So, the viewer is supposed to wonder whether they left her in there and walked out of the house? Or did they do something to kill her?