This going to be a short review.
The second season of Darker Than Black takes place a while after the first season ends. It starts with a young girl whose father is an organization scientist who developed the technology to erase memories. Her brother is a very powerful contractor, however his remuneration is also very severe. The young girl becomes a false contractor after meeting Hei. Hei has changed to the point that he no longer resembles his old self. He is an alcoholic and doesn't eat. He is also out to kill Yin who he once realized he had fallen in love with. I was not a fan of the second season even at first, however after watching the OVA's it made a lot more sense. I would recommend this only after watching the first season AND the OVA's. Other than that it is very similar to the first season, Enjoy!