This story takes place during the human war of resistance against the Arume.

Sachi lost her parents due to the sudden attack of an alien race 7 years ago called the Arume. While living in an orphanage of children who suffered the same fate, a girl with no memory, Ai, suddenly appeared. Who is this girl and what is she so afraid of?

Manga chapter included with DVD#1 of the anime BLUE Drop~Tenshitachi no Gikyoku~