The OVA series takes place after the original series during summer break and Chizuru and Kouta have started dating (at least in Chizuru's mind anyways don't know if Kouta feels the same way just yet) the story follows them though a couple days during the summer break leading up to a summer festival and large fireworks display that ends the series.
The story isn't just about Chizuru and Kouta though as many of the characters from the series show up as well. Chizuru's mom and sister who are there to celebrate the summer festival and make life rough for Chizuru. The twin loli ninja girls are there as well and like always hunting for food. They are always worthy of a good laugh. Chizuru's main rival for Kouta, Nozomu, is there as well but more as a naked back ground character than anything. The interesting thing about the OVA series is that the main plot is the development between Akane and Tayura as he works his way up to asking her out to the summer festival and her relenting and going with him.
All in all if you were a fan of the original series than you will definitely like the OVA because it really is just more of the same lots of fanservice and naughty bits in a pretty mindless romcom anime. I only didn't watch it until now because I put it on the laptop and forgot about it until last week when at work I found out that I forgot to bring my thumbdrive with my other anime on it and only had this OVA to watch. I'm glad I finally did watch it though.
The original anime series when released on DVD included very racy short specials and the OVA series did the same as well. Nothing really new there just more naked and naughty bits.
You can check out a preview for the OVA episodes here:
