I've never seen anything quite like this picture before - it's an amalgamation of suspense, supernatural, and social realist genres and is well worth seeking out.
The film chronicles the various goings-on in the lives of several tenants of a crumbling Seoul highrise, primarily through the eyes of a new leasee who becomes caught up in their morally compromised existences. As the film progresses, we come to realize we know less about our narrator then we thought, and that he may be as capable of evil as the other characters.
To sum this film up is difficult, and I haven't even scratched the surface here. It's beautifully written, acted, and directed (by a first-time helmer no less!) and worthy of attention.