This is a terrific film, never boring. The female lead gave an absolutely terrific performance. She is the mother of this child and has the strength and determination that is riveting to watch. If this wasn't a foreign film I could see her nominated for a best actress.
Without ruining the film, you've read the premise and much of the film is about the mother's indomitable will and spirit to find her daughter and who was responsible for her sudden disappearance. Her determination is unwavering and while there are disturbing truths, she prods on where the police unfortunately have not bothered to pursue for a variety of reasons. If there is a weaker point, i didn't quite grasp the entire reason why that would be the case. But these things do happen.
This is an excellent movie and I highly recommend it to all. As to the tale, it might be just a little bit of a stretch but it's all very reasonably credible and nowhere near what Hollywood tries to pull off and fails. It's not your usual tale you'd expect from Hollywood and Korean films can be a bit dark and often have a bit of a twist, which is what makes them interesting. I cannot even fathom why there is such a low IMDB rating for this film. I can only guess that some don't like subtitles and who are not at all familiar with Korean culture might not appreciate that this is more eastern mentality and that people can actually act like this.