Blue Drop - Maiorita TenshiBLUE DROP ~まいおりた 天使~; BLUE DROP ～舞い降りた天使～; Капля синевы - Явившийся ангел; まいおりた 天使; Blue Drop - Descended Angel

Sachi lost her parents due to the sudden attack of an alien race called the Gosta. 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?