One of Ki-duk's harder to find films. Like most of his post Arirang work, One on One is quite violent and dark, though never gratuitous or without a more contemplative side. I appreciate both eras in Ki-duk's oeuvre, the reason why this one didn't pan out to be a full-on favorite is because of its narrative focus and its somewhat repetitive nature.
A group of vigilantes is hunting down soldiers who were involved in the execution of a young woman. They followed orders, but it's clear they were saving someone's hide rather than act to protect the country. The vigilantes want every soldier to sign a confession, and they want to find out who gave the order for the kill.
There's quite a few soldiers to go through though and the crux of the film is revealed early on, so after a while the torture scenes do get a little repetitive. They also take time away from the character development, which is probably why the ending was as intense as it was supposed to be. It's still an interesting film with more than enough to chew on, it just isn't quite up to Ki-duk's usual standard.