A divorced Korean doctor with sleep problems (including nightmares, which we the audience can't always tell from reality) suspects his landlords - who run the butcher shop under his apartment block - are serial killers after the addled father of the family makes a confession under the influence of drugs during a colonoscopy. (There are too many colonoscopies in this film.) But he doesn't go to the cops because he's a mess himself and he's not sure he can prove anything.
The movie contains several horror movie tropes: nobody is innocent, most people have psychological issues, don't call for backup (or the police) even when they're clearly needed, go into dangerous spaces you know you shouldn't. It's not horror though: Wikipedia lists it as a "psychological thriller."
Another twisty Korean mystery: the critics thought well of it, I didn't like it much. In part because I didn't feel like the parts fit together at the end.