Let me entertain you with my take on the OVA. I can assume you have already watched the whole season Devils' Line.
Are you are here to watch another episode of your favorite characters from this series, know more about Tamaki Anzai and Midori Anzai's past, or both? I'll inform you,that this OVA features Yuuki Anzai's parents. The majority of the OVA is focused on that. However, there are parts that look upon what Anzai and Taira are doing. If you are an avid fan of this series, I recommend you do watch it. It gives you answers to unsolved questions and leaves you a tad bit more satisfied than when you got to the end of the series.It's only about 24 minutes, so it won't hurt to watch it.