The story is clearly outlined in the other well written reviews, so I just wish to add that every actor and actress in this series was top notch. Recently I have been watching several Korean serials and they all show that South Korea has a fantastic stable of skilled performers. Unlike American Method acting, they seem to follow Stanislavski’s system and really become the characters they are portraying. Not two dimensional at any level, you really believe them.
Add to that the superb camera work, thumping soundtrack and clever, non-formulaic script and you have TV which is at last worth watching. If you have not seen this yet, and you’re not too slow to read subtitles, then you really should try this; you will be hooked!