The originality of the idea and its execution is what makes this a fun watch. You won't know where the story is going for at least half of the movie.
I personally would have liked a little more background on Mona Lisa, but it doesn't add or subtract from the plot.
I assumed that the moon would have something to do with her powers, but the story does not dwell too much on the whys and whens.
My favorite part though would have to be Mona's expressions on the flight. That there is some real good acting. I wasn't surprised when I looked Jeon Jong-seo up and found out that she was in "Burning" and "The Call." Both are worth checking out.