This movie is a slow burn mystery that for most will seem to not have much of a payoff or simply be misunderstood. It is very well crafted and done - this really is about the horror of Sara's mental health which is deteriorating - however the film makers left out an important piece of info that surfaces near the middle and end of the movie with the mirrors that explains what her illness is.

Sara (the main character) suffers from Capgras Syndrome a DMS(Delusional misidentification syndrome) -people with Capgras believe that one or more familiar people/loved ones have been replaced by identical strangers and are imposters. This is never mentioned (and for reason as it is the payoff) an illness the old lady in the movie also suffered from. The uncertainty in Sara's mental health does loom large in the movie but is unclear, and maybe the flaw is that it is never brought up, not even at the end, only at most hinted at if the viewer understands this syndrome - most people wouldn't and never have heard of it.

We do see that when Sara goes in the hole to save her son the monster chasing her ends up being her -the film makers literally give this to us and ultimately the most important scene in understanding (literally and metaphorically) what is going on. The Monster is Sara (her mental illness).

At the end of the movie when we feel that maybe Sara is doing better (we see her in a classroom and then interacting with others) -it at most is temporary as she has returned to taking pics of her son -she looks at the pics on the camera with some uncertainty as she is in a room full of MIRRORS.

Overall when you understand what is really happening this is quite an intriguing, suspenseful, entertaining movie.