It's basically about a "killer" (even though he really hasn't directly killed a person) who captures people who he believes "aren't grateful for their lives" and puts them in deadly traps that require some sort of sacrifice, and if they do said task, they live. If not, they die. The films focus on the killer and his backstory, the police who try to capture him, and the people put in the traps.
I really like the films. People call them trashy "torture porn", but really, it's not as gory and violent as people say it is. The story is also very engaging and it features some of the best twist endings in horror history.
