Woman locked away for a long time for a crime she doesn't commit. Comes out later and extracts vengence.
I expected much martial-arts mayhem, but no: she has a cunning plan.
Actually, it's a pretty simplistic plan, and not worth all the fuss made in the movie, but let's ignore that. The good bit is what happens at the end. The last quarter of the film is some deeply twisted moral anguishing, and nicely done. I'd recommend it for that, despite the confusing direction and choppy editing for much of the first half. It's not a nice film, though.