Again: Many spoilers ahead.
At first, I kind of liked this movie - the two lead characters are well played, and so is their relationship as it develops through the meandering memories and the developments in the supposedly real world. It's adequately somber. The twists towards the end were a bit obvious in my opinion - of course she was luring him to the house and setting him up, and of course these were his memories, with the distant guy watching in precisely the way the main character explains at the beginning.
Unfortunately, once you start to think about the plot, it falls apart.
He's a detective, yet he doesn't really look at any of the clues he has taken home (e.g., the yearbook) in any depth until after the case is practically closed. Nice evidence board decoration he had on the wall - he should have perhaps drawn one up for this case too. And when called, he does this oh so old stupid movie mistake of rushing in without telling anybody. When will movie writers finally stop using that?
Her side is even worse. There were signs of her interacting with the outside world, e.g., her drawing of stairs where he believes to have seen her. Was she able to occasionally escape? Apart from that, nothing of her whole back-story was explained; did she or did she not commit any crimes? Had she just been at the wrong place in the wrong time? How much was fabricated by her, how much was honest? What was with that "Mousey" confusion? Who was lying about what now? And finally: If she wanted to be assumed dead to get away from her parents, didn't sending the picture refute that purpose entirely? What is she living on now anyway? If running away was all she wanted, she evidently didn't need this convoluted plan to do so. When our main guy arrived at the house, the doors were already wide open.
I guess you can explain some of those aspects away with "those were just his memories of her memories", but frankly, I think that's just an excuse for a script which was too obsessed with its twists.