When she was 7 years old, what Sorano Fuu (the protagonist) saw in her living room were the lifeless bodies of her beloved parents, who had died at the hands of an unknown culprit.
Taken in by a group of people known as "Tenshi", Fuu asks them why her parents were killed. One of the Tenshi said to the young Fuu, "Your parents were "Dealers", who are people that sell "Hoshi". That is why they were killed."
Her parents were criminal vendors of the illegal-to-sell, taboo Hoshi on a large scale. This truth greatly troubled Fuu, and so she made a vow - to atone for her deceased parents' crimes.
Now, the 20-year-old Fuu is a Tenshi who protects Akatsuki, a town in Cloud.
There was something she saw through the many missions she completed with her colleagues. That "something" is their sad pasts, all of which revolve around Hoshi.
They are still bound by the past; will they be able to break free of the curse of their pasts?
[Translation of the official website's plot summary.]