This is one of the rare mangas I've been reading. Pretty outrageous stuff, and this ova, true to ova traditions, goes all out. What comes to the plot of the ova, it uses basically one chapter of the manga as a core and then alters and mixes in some random elements. The core chapter is not from the very beginning of the manga but a little along the way.

You could say this ova gives an impression of what the whole thing is like, and it reveals at least as much as the manga and most certainly more than the tv series will. This isn't a spoiler sensitive story at least in the beginning and while this ova perhaps reveals what kind of material and relationships you are dealing with, I couldn't honestly claim it would really spoil anything at all.