The story is there, and from what I understand is adapted from a visual novel. I have not read the visual novel, but there is enough tension to properly set up the action so that it's not just wham bam thank you ma'am. That being said, I don't usually expect much of a plot in porn to begin with. However, the confession scene and the "nursing back to health" scenario work very well at conveying the tenderness between Mai and Reo. The story is also devoid of rape, demons from hell, old men, aliens, and all the other banalities you may be prompted to think about when you hear the word "hentai." It was a refreshing change for me.
The soundtrack consists of violins, piano, and in one particular scene some rather err, "celestial" sounding synth choruses. It is nothing to break the mood, although the "climax" of the story may come off as a bit too... grandiose. If there is a weak point in this ero anime, it would have to be the soundtrack as it was overly conspicuous at times and ultimately forgettable.
On the other side of the audio, the two characters Mai and Reo are excellently voiced, if only because they avoid the irritating wailing and groan-worthy pillow talking common in other hentai. The sense of hesitation, embarrassment, and at the same time longing in the dialogue is also carried through superbly. As for the personalities of these two characters, Reo falls on the "tsundere" side of the spectrum while Mai plays the slightly more cool-headed, but persistent-enough-to-take-the-lead "onee-chan." The dynamic between the two plays out wonderfully.
The main enjoyment of this ero anime is not derived from hardcore depravity as it is with other hentai. It is more of a drawn-out, passive warmth than a quick and dirty flash. Of course, this may lead some to call it "boring." And admittedly, it's not for everyone. Yuri fans, however, needn't ask the question any longer—the answer is "Yes! Watch it!"
