I can't say this won't contain spoilers, so I'll just make that a fair warning now.

Now we all know Butterfly Effect with Ashton Kutcher, so I shouldn't have to explain that part. Let me say that first I thought maybe this movie was going to be about the two main leads trying to work together to stop a serial killer. Nope. As the movie progressed I was really struck by how the FL in the future asked the FL in the past to save her dad. All is well until the Seo Yeon (future FL) finds out that Yeong Sook (past FL) killed her mother. It all went down hill from there when the Seo Yeon tried to ignore the Yeong Sook. Desperately trying to break the past, the Seo Yeon knows she can't just ignore the phone calls because it will somehow effect her present. Every little thing that is done has to be the way Yeong Sook wants it, if not then Seo Yeon loses everything she holds dear. So instead of Seo Yeon allowing the past be the past and she saved Yeong Sook, she sacrificed her own freedom and future.

Overall:
It was a great movie. I'd never watched anything like it before and I was very impressed by the acting. Towards the end really kept me on the edge of my seat waiting to see what would happened next.