Writing about what the psychological thriller is about would spoil it completely, so I won't.
Added after 23 hours and 19 minutes
Fetus (2008).
As one magazine told me, there's this guy, Brian Paulin, and he creates the most awful, most disgusting movies imaginable. And his masterpiece is Fetus.
So I watched it. And what? Even Saw looks better. And here, I see bad acting (all of them, without exception - his shaggy-haired friends), including the main role, unskillfully played by Brian himself, and all these horrors didn't really affect me. They take premature babies out of their mother's stomach with a knife and cut off their heads, they rape corpses, they peel the skin off people, and so what? If we talk about Saw, I only watched one part of it a couple of years ago, and I remember the scene at the beginning - a girl is sawed in half horizontally while lying face down. That looked somehow scarier and more interesting.
Brian doesn't use special effects; he creates all the dismemberment with his hands, like in old horror movies. It turns out pretty well, in principle, but sometimes it looks a bit silly.
I'd rather watch a horror movie by Peter Jackson.
Yesterday's "Hide and Seek" was scarier. And definitely much more engaging. However, I might just not like bloody horror as a genre; I haven't watched many of them, so I can't say for sure.