I love the twists in Korean movies. They are obvious once you know them, but the whole trick is that you would have never guessed that this is the way it is going to happen. Here we are given many clues and still it isn't until the very end when everything becomes clear.
I was positively surprised by Hyeon-ju Son playing the main character. I watched him at the same time in The Phone (2015) and thought here his role was much better. The movie is a nice introduction to Seo-Joon Park, who is recently becoming quite popular. If you liked him go and check 'Midnight Runners'.