The missing link follows Jensen in the section where he dissapears and goes off the grid by stoqing away on a boat. This section is largely unexplained in the main game, so it's prime for making some extra content about it.
This DLC looks to be really good, 5 hours claimed probably mean a couple of hours really. The lighting and graphics definitely look improved, and while losing all augmentations might be a bit of a cop out to have you start over for DLC, it looks like a fun way to play the game. Be warned, the video is a little spoilery but it's good if you want to get at all excited for it.