Well, I can't believe my first review will be on a Hentai. Perfect start. Here we have a side story which is centered at Hikaru and her sister in the first two episodes. Even though this Hentai has extremely disturbing scenes and controversial taboo themes no one dares to mention, what surprises viewers the most(if they can get past the disturbing sh**) is character development. Everything is for a reason. Desire, lust, pain, despair. You see it all in the characters throughout the story. Hikaru's sacrifice, and worry for her sister. The doctors bizarre, yet helpful act. Then you can see rage, surrender. I'm not a fanof yuri (at all, I hate it) but something in that scene is very emotionally perceived how he connects the sisters.

The last episode is a fun one. If not the most humorous of NSN.
It depicts something as of a utopia, a fairy tail kind of setting for both Ryuji and Ren.
It's very amusing seeing their life together in that village.
The sex scenes are outstanding as always, with everything that NSN has to offer, you know what i mean if you watched the previous series.

All in all, I don't want to spoil a perfect ending.
It gets a 9 from me~

^_^