Let's get the details out of the way, before I sing my praises. Oh Dae-Su finds himself kidnapped and imprisoned in a hotel-esque room for 15 years. He teaches himself to fight (by beating the hell out of a brick wall!), and slowly begins to go crazy. He's then hypnotized, and wakes up on the roof of a building. He has a limited amount of time to find the people who imprisoned him, find out why they did it, and then exact revenge.
OKAY, now you've got the gist of the plot, allow me to rant and rave about how amazing this film is. The plot (taken from the manga of the same name) is out of this world. It's amazing that Hollywood didn't snap it up sooner (though unfortunately, Hollywood are planning to butcher it with a remake). It's twists and mind boggelers galore. Guaranteed to disturb and disgust you in parts, and leave you touched in others. Everything is brought together with strong characters, amazing music, a top-notch storyline, and most importantly, great acting. Every person feels like they were born to play their role.
I can't stress enough how much you need to watch this film. Pretentious critics like to sling mud at Oldboy, but none of it sticks. Despite over-hype and the occasional "well it wasn't THAT amazing" review, this film stands its own against any other film out there. I would go as far as to say it's my favourite film of all time. It's epic in a subtle way, and if you're not left mesmerized (albeit due to some confusion) by it, watch it again. I didn't notice the beauty of it all until its second viewing. A must see for any fan of intelligent films.