It begins when Shou Hamura comes to Yuusei Academy, from there it branches out into the truth behind his past, and his twin brother, Kai's as well (though they both have the same name, they are referenced differently). It also portrays shonen-ai on the behalf of Kai and Nagi (his supposed best friend).
This story is considered as shounen-ai for the fact that Nagi and Kai are bestfriend and have a relationship.

Shou Hamura and Kai Misonou are twin brothers,( Shou is the older twin brother while Kai is the younger twin brother), In the beginning, when Shou was mentioned that his lost twin brother, Kai was in the same school, Shou went looking for Kai first, only to find out that Kai had completely forgotten about his older twin.

Shou was adopted from a wealthy family, while Kai’s family is controlled by Ran, who is Reiya’s retainer, which leads to the fact why Kai knows Ran.

I guess Shou comes to Yuusei Academy for he is good at kendo, Shou met his bestfriend Naoto Aoki, while playing or practicing, but when the group had a mission, they fought Naoto and discovered that he is a black Wing. This is also like what happened to Kai's bestfriend Nagi, whom they also fought and they know that Nagi became a Black Wing.

..For me..
The OAV is quite a bummer, they just have 2 OAV and i really wish we know what happened in the last episode.