Kwon Yoo (Ji Chang-wook) is introduced to us as "Captain," a video game player who is an excellent leader and takes extraordinary care of his team. This relatively short introduction is then followed by him finding a phone at the internet café and returning it to a woman's apartment. When he wakes up the next morning, he's arrested for the rape and murder of the phone owner. It's a successful frame-up, and he's quickly jailed for life.
I'll spare you the nasty details of prison, suffice to say that after a few months he manages to break out to try to clear his name. He does surprisingly well, but there's a national man-hunt for him - he only stays on the loose with the aid of his former video game team mates. He's very surprised (why?) that none of them look remotely like their avatars (perhaps because he's young and handsome himself, and does look like his avatar).
The movie is violent and nasty, and a little shaky about its tone: sometimes it thinks it's a comedy (but it never sticks with that for more than a couple minutes), sometimes action, sometimes revenge. Mostly it's a revenge flick, but its emphasis on loyalty within Kwon Yoo's team is kind of appealing (if not entirely believable). I enjoyed it, but I'm not sure it's going to work for many people with its combination of tonal inconsistency and nasty violence.