I bought what I thought was a DVD of the Korean film _Natural City_ at a science fiction convention. Several weeks later, I got around to watching it, and discovered that instead of a Korean film about cyborgs, I had a Japanese film about volcanic eruptions destroying civilization and annihilating most of humanity.
I wasn't sure I'd like the movie when I realized, about one minute into it (I can read hangul, but know hardly any kanji) that this wasn't the film I thought I'd bought. For the first 15 minutes or so I watched it with my finger hovering over the "stop" button on the remote, but after that amount of time I decided that I wanted to see the whole film.
The main characters of the film are Teru and Ako, two teenagers who were on a train going to Tokyo from Kyoto when something happened, causing the train to crash in a tunnel, killing almost everyone on it except for the above-mentioned two, and a boy named Nobuo, who has gone insane. For a while I wasn't sure if this was a general catastrophe, or just a train wreck; after a bit, it is revealed that there indeed has been a cataclysm, and not many are left alive.
The main theme of the film is that of the conflict between conforming to authority and individualism: this is shown early in the film when Nobuo screams at his badly-injured schoolteacher
"You're the adult here! You've got to help me!" The suggestion is that Nobuo in part loses his sanity because there's no one there to tell him what to do.
This theme is further developed through the course of the story (no spoilers here!). I'll go so far as to say that the film does not have a happy ending, but that the ending is satisfying (at least to me).
I liked this film better than I expected I would, and will probably watch it again sometime; I therefore recommend it.